Multimaster with robot having multiple computers not able to publish messages

[Dave-van-der-Meer]

Issue

In my setup, I can subscribe to topics, but I can't publish into the topics created by a remote master. I can see the topics through rostopic list but when I publish messages, they are not received on the other end.

Setup

I have the following setup:

• one PC with its own ROS master
• one robot with two single-board computers
  • Raspberry Pi with its own ROS master
  • Nvidia Jetson with the ROS master exported to the Raspberry Pi

I want to connect the Raspberry Pi to my PC with the Multi-Master system, but I read in the comments of this question that the ROS_IP and ROS_HOSTNAME could create some issues.

Questions

How can I configure my system in a way that all 3 devices can communicate together? Do I need to run its own ROS master on the Nvidia Jetson? How do I need to have the ROS_IP and ROS_HOSTNAME set in order to make it work?

[atiderko]

Hi Dave,

as you have already correctly recognized, the best solution would be to start own ROS master on the Jetson as well. However, if this is not possible, you can run the master_sync on the PC with the parameter sync_remote_nodes:=True.

In most cases you don't need to set the ROS_IP or ROS_HOSTNAME. But in some scenarios you have a host which can't resolve the hostname of your PC. In this case you can use ROS_IP to force ROS to use IP addresses instead of hostnames. So the robot can recognize your PC. Don't forget to set the IP also in ROS_MASTER_URI!

The advantage of using the ROS_IP is that no host resolution is required for communication. The disadvantage, however, is that ROS now only communicates via this IP. If this is e.g. a WiFi interface and the connection is lost, there is also no local communication!

[Dave-van-der-Meer]

Hi, Thanks for the confirmation. In the end, I tried to keep the setup as-is so that the Nvidia Jetson exports the ROS master to the Raspberry Pi and I could make the connection to my PC work by exporting the ROS_IP of the PC and by exporting the ROS_IP of both the Jetson and the Raspberry.

But I do not export the ROS_HOSTNAME on my PC.