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0 votes
asked by (31 points)
Dear all,

I try to discover running X-Plane instances by means of the "BECN" message sent out via UDP multicast. this works well on Linux, but I have no success with Windows machines running X-Plane.

I can see the UDP-multicast messages of several Linux machines using Wireshark on Linux, but I don't see any UDP-messages from the Windows machines.

Is Windows 10 somehow blocking UDP-Multicast messages? How can I make those messages come through!

I'm running out of ideas, so any help is very much aapreciated.



1 Answer

+1 vote
answered by (20 points)
Are your windows machines listening via wireless?

BECN message are UDP multi-cast packets which are not transmitted over wireless connections unless the wi-fi router is configured to enable Multicast-Unicast Conversion (and most home router's don't have this functionality).
commented by (31 points)

Kudos Avacii!

It really works as soon as I connect both computers to the wired connection, it works like a charm.

Do you have a pointer where I can read about the background of this behavior?

I just don't understand, why this does not work with one computer being Windows 10. When both computers run Linux, it just doesn't matter whether it's WiFi or a cabled connection. It just works. So, if you have further information on that, I'd like to learn about it.

For the moment being, I'm glad to have a workaround.

So Thanks again!

commented by (20 points)
edited by

There is also a "feature" on Windows whereby it doesn't start listening to a multi-cast group until one packet has been sent to the group. It can be 1 byte of 0x00.

My C# Xamarin code has the following:

if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))   //UWP only successfully listens to a multicast group once a packet has been sent.

                m_Client.Send(new byte[1] { 0x00 }, 1, m_MulticastAddress);

Ref: No multicast over wi-fi. The punchline is as followed. Imagine 2 clients, one connected at 11Mbps, the other at 54 Mbps,  How can a router transmit the same packet simultaneously at 2 different speeds? Hence the need for multicast to Unicast. 
This paper from the IETF details the problems: