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04:57 AM

Topic 4 — Fleet Communications & Inter-Agent Messaging

Inter-robot communication is the backbone of all multi-robot systems. This topic explores fleet networking, message passing, reliability/safety, and the tools available in ROS2, DDS, and modern IoT/cloud frameworks. Robust comms are required for everything from map merges to market-based auctions, and are crucial for scaling robot teams.

4.1 ROS 2 DDS Networking & QoS​

  • ROS 2 decentralizes communication using DDS (Data Distribution Service).
  • Topic sharing: Publish/subscribe model works across a fleet if domain IDs, namespaces, and QoS are configured correctly.
  • Quality of Service (QoS):
    • Reliable vs. best-effort (use reliable for mission-critical messages; best-effort for high-rate sensor streams)
    • History, durability, lifespan parameters for tuning performance and reliability
  • Practical settings: For large fleets/tight safety, prefer static discovery, bounded resource pools, and explicit heartbeats/timeouts for liveness.
  • Comms patterns: Broadcast (all robots), peer-to-peer (pairs/teams), relay (one robot as gateway to the cloud or web dashboard)

4.2 API-Based Multi-Robot Control​

  • REST APIs: Human or backend services can assign or query robot state and tasks via simple HTTP.
  • WebSocket: Real-time dashboards and bidirectional cloud comms for remote fleet management.
  • MQTT & brokers: Message brokers (e.g., Mosquitto, HiveMQ) allow lightweight, scalable publish-subscribe comms for robot-to-robot and robot-to-cloud coordination.
  • Edge/cloud hybrid: Some robots operate with edge-only comms, others relay state/data to cloud dashboards or centralized planners.
  • Broker relays: Allow communication across network/firewall/domain boundaries, and log all comms for security audits.

4.3 Security & Identity​

  • Authentication: Each robot must have secure ID (cert/key, pre-shared auth, or dynamic credential exchange).
  • Encrypted channels: TLS for DDS and MQTT, or VPN overlay for entire fleet network.
  • Access roles: Partition robots by capabilities (scout/worker/carrier), restrict commands based on role/user, and log all high-stakes commands.
  • Fleet-level safety: Monitor for spoofing, replay, or compromised bots. Real-time network integrity checks with alerts/escalation protocols.

Diagram: Fleet Network Schematic​

+----------+   TLS  +----------+   MQTT+REST   +----------+
|  Robot 1 |<-----> | Broker/  | <--------->   |  Cloud/  |
|          |        | EdgeHub  | (encrypted)   | Operator |
+----------+        +----------+               +----------+
   |  DDS pub/sub    ^   ^      Peer relays         |
   +----------+------+   |           ----------------+
              |         +---+  QoS-partition     |
         +----------+   |   |                       |
         |  Robot 2 |<--+---+---...---Robot N ...---+
         +----------+

Lab: ROS 2 Fleet DDS Domain + Reliability Testing​

Goal: Set up a multi-robot ROS2/Gazebo fleet. Vary comms settings and demonstrate:

  • Topics propagate between robots
  • Single-robot failures don’t halt fleet; QoS tuning affects message loss/delay
  • (Optional) Relay robot/REST/MQTT dashboard to observe state and queue comms

Tasks:​

  1. Launch two+ robots with separate and shared DDS domain IDs.
  2. Configure topics for both reliable (commands/goals) and best-effort (sensor streams) QoS.
  3. Log:
    • Message propagation delay and loss (packet drops, QoS mismatch)
    • Reaction to simulated disconnections or network partition
  4. (Optional) Connect edge broker or cloud dashboard for remote monitoring/control via API

Deliverables:​

  • DDS and comms config files
  • Logs of message passing, failures, timing, and dashboard connectivity
  • Short report: Fleet comms bottlenecks, lessons on ROS2 networking for large-scale deployments

Summary​

Any distributed autonomy depends on the speed, reliability, and security of fleet communication. Getting QoS and network topology right unlocks safe, resilient, and scalable robot teams for all higher-level behaviors, planning, and mission assurance.