Zenoh Crash Course

In the context of ROS 2, using the Zenoh RMW (rmw_zenoh_cpp) simplifies connecting multiple devices across different subnets.

Overview and Concepts

When using Zenoh with ROS 2, the network architecture is slightly different from traditional DDS:

  • Router (zenohd): Think of this as a ros2 daemon running on your host. It handles network discovery and host-to-host communication. Typically, you run one router per host. Routers must be configured to talk to each other across the network.

  • Sessions: Every ROS 2 node creates a Zenoh “session” that communicates through the local loopback interface (lo). The router then picks up this traffic and handles the host-to-host routing.

The router is required for the initial discovery phase. In the common case, even if the router process dies, the established connection between ROS 2 nodes will persist, though new nodes will not be able to discover each other until the router is restarted.

For deeper technical details, refer to the official rmw_zenoh documentation.

Workspace Configuration

To enable Zenoh, you need to set the appropriate environment variables. RTW expects these to be defined in your workspace configuration.

The best solution is to set Zenoh variables per specific workspace either at the creation with rtw workspace create or later using the rtw edit command. In both cases the flag is --env-vars RMW_IMPLEMENTATION=rmw_zenoh_cpp export RUST_LOG=zenoh=warn,zenoh_transport=warn. If you want to use Zenoh to connect to a remote computer append ZENOH_CONNECT_IP=192.168.28.28.

If you want to use zenoh as default in your system open your ~/.ros_team_ws_rc file and uncomment the following lines.

export RMW_IMPLEMENTATION=rmw_zenoh_cpp
export RUST_LOG=zenoh=warn,zenoh_transport=warn
# Optional: Set a default target IP to connect to automatically
export ZENOH_CONNECT_IP=192.168.28.28

Once added you will have to source ~/.bashrc again or better, open new terminal and then choose the workspace using rtw ws <ws_name>.

Starting the Router

Every machine participating in the Zenoh network needs access to a Zenoh router (rmw_zenohd). RTW provides the rtw-zenoh-router alias to manage this.

Local-Only mode If you are developing locally and do not need to bridge to an external device, start the router without arguments in one terminal:

rtw-zenoh-router

What it does: This launches the router in listen mode, listening to all interfaces for incoming connections, but does not attempt to connect to other routers.

Connected Mode If you need to connect your local machine to an external device also running a zenoh router:

rtw-zenoh-router 192.168.28.28

If you need to connect your local machine to an external device also running a zenoh router, the script will automatically connect if you either:

  1. Defined ZENOH_CONNECT_IP in your workspace configuration and run rtw-zenoh-router without arguments. How to set this variable per workspace or system-wide check the explanation above.

  2. Explicitly pass the IP as an argument (this overrides the workspace variable, setting ZENOH_CONFIG_OVERRIDE='connect/endpoints=["tcp/<IP>:7447"]'):

Verifying the Connection

If you want to see detailed connection logs, you can increase the RUST_LOG verbosity in your terminal before running the script:

export RUST_LOG=zenoh=info,zenoh_transport=info
rtw-zenoh-router 192.168.28.28

When using Zenoh, take care that the ros2 commands are executed differently. More about this check the ROS 2 Daemon Management documentation.