The Po C is here: https://github.com/shykes/docker/tree/extensions/extensions/simplebridge -- please keep in mind this is seriously alpha code and some corners were cut to accomodate our Po C.To answer the extensions question; yes, it is being worked on as a separate project.
Is there any working code that shows as an extension?
And are these extensions planned to be late-bound, out-of-process and delivered as a container?
Here's a basic diagram about how the networking itself operates: The following includes a description of the interface and its name.
The sub-bullets provide practical examples of these components with the existing implementation of Docker as an example.
For a multi-tenant environment (i.e people that don't trust each other, but use the same infrastructure/cloud to deploy docker), then a very simplistic workflow like this would be needed.