ex-5.md

Exercise 5 - Discovery via UDP multicast

Even if not enabled by default in rmw_zenoh, Zenoh can be configured for discovery over UDP multicast in several aspects:

• For the routers to discover each other and to automatically inter-connect with each other.
• For the ROS Nodes to discover each other and to automatically inter-connect with each other. This allows a router-less deployment.

Warning

If your host runs MacOS, your Docker probably doesn't router the multicast traffic, even if the container is using the --net host option. Therefore you won't be able to participate to this exercise.

Routers discovery

Each attendee will run 1 container with the Zenoh router and a publisher and a subscriber on the same topic. The routers will be configured to discover each other over UDP multicast and to automatically connect to each other.

Configuration

Edit your zenoh_confs/ROUTER_CONFIG.json5 again to:

• remove all endpoints in connect.endpoints configuration:

    connect: {
      endpoints: [
      ],
    },

• configure the scouting.multicast setting as such:

   multicast: {
      /// Whether multicast scouting is enabled or not
      enabled: true,
      /// The socket which should be used for multicast scouting
      address: "224.0.0.224:7446",
      /// The network interface which should be used for multicast scouting
      interface: "auto", // If not set or set to "auto" the interface if picked automatically
      /// Which type of Zenoh instances to automatically establish sessions with upon discovery on UDP multicast.
      /// Accepts a single value or different values for router, peer and client.
      /// Each value is bit-or-like combinations of "peer", "router" and "client".
      autoconnect: { router: "router" },
      /// Whether or not to listen for scout messages on UDP multicast and reply to them.
      listen: true,
   },

Run

Now you can run the following commands in your container:

• ZENOH_ROUTER_CONFIG_URI=/ros_ws/zenoh_confs/ROUTER_CONFIG.json5 ros2 run rmw_zenoh_cpp rmw_zenohd
• ros2 topic pub /chatter std_msgs/msg/String "data: Hello from <YOUR_NAME>" - (replacing <YOUR_NAME>)
• ros2 topic echo /chatter

Bonus

Try running rmw_zenoh Nodes without any router in 2 different configurations:

1. the Nodes shall only discover the other Nodes running on the same host
2. the Nodes shall discover all Nodes running in the LAN

Solution 1

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• Copy the file zenoh_confs/DEFAULT_RMW_ZENOH_SESSION_CONFIG.json5 as zenoh_confs/SESSION_CONFIG.json5

• Edit zenoh_confs/SESSION_CONFIG.json5 and just set scouting.multicast.enabled to true as such:

   multicast: {
      /// Whether multicast scouting is enabled or not
      enabled: true,
      /// ...
   }

• Then run:

  • ZENOH_SESSION_CONFIG_URI=/ros_ws/zenoh_confs/SESSION_CONFIG.json5 ros2 topic pub /chatter std_msgs/msg/String "data: Hello from <YOUR_NAME>"
  • ZENOH_SESSION_CONFIG_URI=/ros_ws/zenoh_confs/SESSION_CONFIG.json5 ros2 topic echo /chatter

Solution 2

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With previous configuration the Nodes on different hosts didn't discover each other because they're configure to listen for incoming connections only on the localhost interface. To enable inter-hosts discovery and connection, we need to change this.

• Edit the same zenoh_confs/SESSION_CONFIG.json5 and set the listen.endpoints configuration as such:

  listen: {
    endpoints: [
      "tcp/[::]:0"
    ],
  },

  Here [::] means any IPv6 or IPv4 interface, and 0 means the OS will choose an available port number.

• Then run:

  • ZENOH_SESSION_CONFIG_URI=/ros_ws/zenoh_confs/SESSION_CONFIG.json5 ros2 topic pub /chatter std_msgs/msg/String "data: Hello from <YOUR_NAME>"
  • ZENOH_SESSION_CONFIG_URI=/ros_ws/zenoh_confs/SESSION_CONFIG.json5 ros2 topic echo /chatter

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