it's a bit sideways!

* postcard-rpc is a point to point protocol
* poststation is a reverse proxy, it helps you do 1:N, so talk to one server (Poststation) that talks to N postcard-rpc servers/devices
* ergot is more of a "message bus" or "network stack" - it allows you to have a whole network of devices, where any device can be a server, client, both, etc.