[Discourse.ros.org] [Next Generation ROS] No communication if no gateway is set

[David!! via ros-users]

Hi,
I ran into trouble with a simple network setup consisting of two computes connected via a switch. I noticed ros2 will only send data in case a gateway is set (the gateway address doesn't need to be reasonable - for example I just set it to the ip of the switch).

This leads me to the question. Why do I need to set a gateway?





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[David!! via ros-users]

What version of ROS2 are you using?





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[David!! via ros-users]

I'm on the current master branches.





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[David!! via ros-users]

Any ideas someone ?
This issue isn't that fatal but I'd like to know if I'm doing anything wrong or got a wrong understanding of the system.





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[David!! via ros-users]

Could anyone of the ros2 team say if this is intended behaviour (or at least determined by the middleware) or if this is a bug?





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[David!! via ros-users]

Maybe nobody else has answered since nobody knows the answer to your question out of the head. I quick [Google search](http://lmgtfy.com/?q=udp+multicast+gateway) seems to indicate that a gateway might be required for udp multicast which are being used for the DDS discovery.





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[David!! via ros-users]

Slightly related to this, you may want to make sure that multicast is
enabled on your NIC. I've noticed that on some Linux machines even the
local communication doesn't work wo/ the multicast being explicitly
activated on the loopback adapter.
See here: https://github.com/eProsima/Fast-RTPS/issues/63





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[David!! via ros-users]

Thanks for your answers.
@dirk-thomas Sry didn't wanted to sound rude.
As a user it is at the moment quite hard to tell if some behaviour is intented, a bug, or if I'm simply using it the wrong way

Regarding the gateway behaviour:
I was just really irriated because this kind of setup might occur sometimes in robotic systems.  


@karsten Some of these phenomenas I already noticed myself. The loopback interface communication without having any other interfaces activated never worked for me regardless of using FastRTPS or OpenSplice.





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[David!! via ros-users]

Just to make sure, this is not necessarily a problem of Fast-RTPS. I was
giving the link to the issue because it also shows you the steps needed to
enable multicast on the lo device.





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