ROS2 Large Messages: Humble, Rmw_zenoh, And Network Tips

Aug 22, 2025 by RICHARD 57 views

ROS2 Humble and rmw_zenoh for Large Messages: A Deep Dive

Tackling Large Messages in ROS2 with Humble and rmw_zenoh

Hey everyone, let's dive into a common headache for ROS2 users: handling large messages. Specifically, we're going to explore how to get ROS2 Humble and rmw_zenoh to play nice when your data packets exceed the typical size limitations, say, beyond 800kB. It's a scenario many of us face when dealing with sensor data, high-resolution images, or complex point clouds. This article is heavily inspired by a user's experience (as you'll see below), so we will try to give you a practical guide on how to make it work. So, if you're wrestling with similar challenges, you're in the right place! We'll cover the setup, potential pitfalls, and some workarounds to keep your ROS2 applications humming smoothly, even when dealing with hefty data payloads.

The Core Challenge: Large Messages and ROS2

The default configurations in ROS2 are often optimized for smaller message sizes. When you start pushing the limits, things can get wonky. You might encounter transmission failures, dropped messages, or performance bottlenecks. The rmw_implementation, which is the ROS middleware layer, is crucial for message transport, and each implementation (e.g., rmw_fastrtps, rmw_cyclonedds, and rmw_zenoh) handles large messages differently. Some are more robust than others. rmw_zenoh is designed for efficient data distribution, which makes it a great option for those who need to deal with large messages. But still, you have to set it up properly. One key factor is the network configuration, which directly impacts how well your ROS2 nodes communicate. VPNs, mobile routers, and cloud environments introduce extra layers of complexity, influencing latency and bandwidth.

The User's Dilemma: A Real-World Scenario

Let's look at a user's issue: A ROS2 user, using Humble and rmw_zenoh, was struggling with large messages. They set up the following:

Machine 1: Connected through a VPN via a mobile router.
Machine 2: An Azure VM connected to the internet.

The user was having trouble transmitting messages of a size that exceeds 800KB. This user case clearly shows how network setup, along with message size, can cause problems. The user's setup highlights the need for optimizing the network configuration and message transport. It's a classic case of needing to balance bandwidth limitations with the demands of your application.

Diving into `rmw_zenoh` and its Configurations

rmw_zenoh is a powerful ROS2 middleware implementation that utilizes the Zenoh framework for data distribution. Zenoh is designed for low-latency, high-throughput communication, making it a great choice for applications that need to handle large messages. However, to fully harness the potential of rmw_zenoh, it's crucial to understand and properly configure its settings. We need to look into the configuration parameters and how to tune them for optimal performance.

Key `rmw_zenoh` Configuration Parameters

zenoh.locator: Specifies how Zenoh peers find each other on the network. You can use various locator strategies, such as UDP, TCP, or multicast. The best choice depends on your network setup. In the user's case, the VPN and mobile router introduce complexities. UDP might be blocked by the firewall, while TCP might be a better fit.
zenoh.multicast.interface: If using multicast, this setting defines the network interface to use for multicast traffic. Incorrect settings can cause connectivity issues.
zenoh.congestion_control: Allows you to enable or disable congestion control mechanisms. Depending on the network conditions, you might need to experiment with enabling or disabling these features to improve performance.
zenoh.buffer_size: This sets the internal buffer sizes used by Zenoh. Adjusting these buffers can help manage message flow, especially for large messages. Increasing this size can help to handle a large message. Keep in mind, you need enough memory to handle those buffers.

Configuring `rmw_zenoh`

Configuration settings are typically passed through environment variables. You can set these variables before running your ROS2 nodes. Here's an example of how to set the zenoh.locator to use TCP:

export RMW_IMPLEMENTATION=rmw_zenoh
export ZENOH_LOCATOR=tcp

You can also configure specific locator addresses:

export ZENOH_LOCATOR=tcp://machine1_ip:7447,tcp://machine2_ip:7447

Experimenting with these configurations is often necessary to find the best settings for your network environment. We are going to deep dive into this below.

Network Setup: The Foundation for Reliable Communication

Your network configuration is the backbone of your ROS2 communication. The choices you make here will significantly impact the performance and reliability of your large message transfers. Let's examine the key components involved in the user's setup and explore ways to optimize the network.

Understanding the Network Components

VPN: A Virtual Private Network (VPN) encrypts and routes your network traffic through a secure tunnel. While VPNs enhance security, they can also add latency and reduce bandwidth. The choice of VPN protocol and server location influences performance. The VPN is on Machine 1, which can be another point of bottleneck.
Mobile Router: Mobile routers provide internet connectivity via cellular data. They are convenient but often have limited bandwidth and higher latency compared to wired connections. Cellular networks' performance fluctuates based on signal strength and network congestion. This is a major factor in the user case.
Azure VM: The Azure Virtual Machine (VM) is hosted in the cloud. Cloud VMs typically offer high bandwidth and low latency, but the connection to the internet and any firewalls configured on the VM can affect performance.

Optimizing the Network Configuration

Here are some practical steps to optimize the network for large message transfer.

Choose the Right VPN Protocol: Experiment with different VPN protocols (e.g., OpenVPN, WireGuard, or IPSec) to see which performs best in your environment. Consider protocols that prioritize speed and have low overhead. Be aware of the security/performance trade-off.
Optimize VPN Server Location: If possible, choose a VPN server location that's geographically closer to your Azure VM to reduce latency. A lower ping time always helps improve the overall performance.
Monitor Bandwidth and Latency: Use tools like ping, traceroute, and iperf to monitor bandwidth and latency between your machines. Identify any bottlenecks in the network. This helps to ensure you are optimizing everything to get maximum performance. Check the network connection and throughput constantly.
Configure Firewall Rules: Ensure that your firewall rules on both the VPN and the Azure VM allow the necessary traffic for Zenoh. Open the required ports for TCP or UDP, depending on your zenoh.locator configuration.
Prioritize Traffic: Consider using Quality of Service (QoS) mechanisms to prioritize ROS2 traffic, especially if the network is shared with other applications. This can help reduce the impact of network congestion.
Optimize Mobile Router Settings: If possible, adjust the mobile router's settings to improve performance. Make sure the router has a strong cellular signal and is not overloaded with connected devices. Consider using an external antenna if signal strength is poor. You have to make sure everything from this end is working as expected.
Test Different zenoh.locator Configurations: Experiment with different zenoh.locator settings (e.g., TCP, UDP, or multicast) to find the best fit for your network setup. TCP is often more reliable over VPNs, while UDP might be faster if not blocked by firewalls.

Practical Steps: Troubleshooting and Solutions

Let's break down some practical steps you can take to troubleshoot and solve large message transmission problems with ROS2 Humble and rmw_zenoh.

Step-by-step Troubleshooting Guide

Verify the RMW_IMPLEMENTATION: Ensure that you've correctly set the RMW_IMPLEMENTATION environment variable to rmw_zenoh on both your publisher and subscriber nodes. This seems obvious, but it's a common source of errors.
```
export RMW_IMPLEMENTATION=rmw_zenoh
```
Check for Network Connectivity: Use basic network tools to ensure that your machines can communicate with each other. A simple ping test can reveal whether there's a basic network connection. Also, make sure you have the correct IP address.
```
ping <machine_ip>
```
Monitor Network Traffic: Use tools like tcpdump or Wireshark to monitor the network traffic. This will help you identify whether messages are being sent and received, and it will also let you inspect the contents of the messages.
```
sudo tcpdump -i <interface> -n -v host <machine_ip>
```
Examine ROS2 Logs: Check your ROS2 node logs for any error messages or warnings related to message transmission. These logs often provide valuable clues about what's going wrong. Increase the verbosity of your logging as needed.
Test with Smaller Messages: Start by sending smaller messages to confirm that the basic communication is working. Then, gradually increase the message size to identify the threshold at which transmission fails.
Test with a Simple Example: Create a simple publisher and subscriber pair that sends and receives a large custom message. This will help you isolate any issues specific to your application's messages.

Potential Solutions and Workarounds

Adjust rmw_zenoh Configuration: As discussed earlier, adjust the zenoh.locator, zenoh.congestion_control, and buffer sizes to match your network's characteristics. Experiment with different settings to optimize performance.
```
export ZENOH_LOCATOR=tcp://<machine1_ip>:7447,<machine2_ip>:7447
```
Increase Message Size Limits: While rmw_zenoh is generally good with large messages, you might still need to adjust internal limits. You can try to increase buffer sizes or other parameters within the rmw_zenoh configuration.
Optimize Message Serialization: Ensure that your messages are serialized efficiently. Avoid unnecessary data copies and use optimized data types. This can reduce the overall size of your messages and improve transmission times.
Implement Chunking: If possible, consider breaking down your large messages into smaller chunks. Publish these chunks separately and reassemble them on the receiving end. This can help to improve reliability, especially on unreliable networks.
Use Compression: Compress your messages before sending them. This can significantly reduce the size of the data and improve transmission times. However, compression adds some overhead, so choose a compression algorithm that balances compression ratio and performance. ROS2 has built-in support for compression.
Consider a Different RMW Implementation: If rmw_zenoh does not meet your needs, consider other RMW implementations. Though less performant, rmw_fastrtps can still be a solid choice.
Network Segmentation: For complex setups, segment your network to isolate ROS2 traffic and minimize congestion. This can involve using VLANs or separate subnets.

Conclusion

Dealing with large messages in ROS2 Humble with rmw_zenoh can be challenging, but by carefully considering the network setup, configuring rmw_zenoh properly, and employing the troubleshooting steps and workarounds described above, you can significantly improve your application's performance and reliability. The user's case study highlights the importance of understanding the entire system, from the network layer to the RMW implementation. Remember to monitor, test, and iteratively refine your configurations to achieve the best results for your specific use case. Happy ROSing!