Hello Stuart,
The thinking here is if I want to send a message to a station, VK4BWI-0, if it's just to that station, I'd address it to "VK4BWI-0" as normal. However, supposing a few of us in Brisbane WICEN had a little chat group on our digipeater network, and we wished to have a common UI frame destination address that we all listen for to receive real-time messages.
So you're looking to use unconnected UI frames to communicate to multiple people at the same time? Are you looking to have those remote stations listen and *only* selectively show the traffic either to their own callsign+ssid or to a specific "multicast" group?
In my area, we have a weekly UI net where different HAMs essentially send beacons along a 7-hop digi path to chat. Everyone monitors the channel in a promiscuous method to hear everyone's traffic. It's not an ideal system as we see all the other traffic on the channel as well but it does work well enough. If you wanted to filter on this traffic to only your net, you could have everyone send their traffic to a specific destination across your digipeaters and filter all promiscuous traffic to a specific destination address, say "vk4cht-0". If you're looking for a guaranteed delivery transport, you could do something at the application layer to put a sequence number in the UI payload to identify missing packets and re-request a re-transmit. This is something that D-RATS had developed years ago. That could become a challenge in a lossy multi-point network but it it works fine when someone is acting as a net control.
In the TCP/IP world, Pragmatic General Multicast (PGM) attempts to solve this problem:
https://en.wikipedia.org/wiki/Reliable_multicast --David KI6ZHD
