How to setup ROS for multi-robot domains (roscores, namespaces for nodes, tf frames)

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For simulation you have a very complete answer here about how to manage node namespaces and tf issues.

Working with real robots makes it harder. I suggest using a roscore in each robot, so that all data is private to each robot. In order to share certain topics, you can use packages such as ros-rt-wmp, which provides a wireless communication protocol that propagate selective topics to other roscores.