[Detnet] L2/L3 model?

[Erik Nordmark <[e-mail]>]

After the BoF I realized there was one thing we didn't talk about which 
is what combined L2 and L3 topologies that folks have in mind.
It is true that from a packet forwarding perspective both L2 and L3 have 
queues and clocks, but the interaction with the control plane and the 
approach might be different for different forms of combinations.

First of all we have 6TISCH which is an L3-only network.

But in combined L2/L3 networks we could have at least
  - interconnecting L2 islands using L3
  - arbitrary topologies with mixtures of L2 and L3 forwarding devices

A suggestion (at the mike during the BoF) was to consider pseudo-wires. 
That might make sense when interconnecting L2 islands.
But with arbitrary topologies one could end with with a path that as a 
mixture of bridges and routers e.g.

      Sender - B1 - B2 - R1 - B3 - B4 - B5 - R2 - R3 - Listener

Are there use cases that result in such topologies/paths?

Would one need one controller which is aware of both the L2 and L3 
devices and can pick paths (with resources) that include both?
(Typically we separate the layers thus we might have a PCE which sees 
the L3 topology but not the L2 devices in between the routers.)

I think it would be good to explore the combined L2/L3 use cases and 
models in more detail.

    Erik