Simplify light switch use case and conform better to current practice

[esko.dijk@…]

Based on last WG meeting discussions and input; it is proposed to simplify the lighting use case and conform better to current/expected practice in LLNs. Specifically:

1. Use a simpler alternative to MLD (as MLD is currently not designed to operate in LLN environments; nor expected to be used in typical LLN deployments). MLD will still be listed as an option along with others. The simpler alternative is preconfiguration in 6LBRs what multicast traffic is filtered and which not.
2. Light switch will CoAP multicast directly (not via Proxy) to make the basic case more clear. (Using proxy is an optional extension of the basic use case.)
3. Explain how the light switch deals with dropped packets. Current solution envisaged is that lights do not send CoAP responses to multicast. Light switch relies on routing (e.g. MPL/Trickle-multicast for LLNs) for reliable-enough delivery.
4. Remove the DNS resolution step in the basic use case. Using DNS is optional and should be presented as such.

[esko.dijk@…]

Done in upcoming groupcomm-04