Add _nmos-netctrl._tcp DNS-SD advertisement/discovery procedure

[garethsb]

Resolves #42.

This must presumably be optional for v1.0.x, but could be made required for v1.1+.

This PR is obviously heavily based on AMWA IS-04 Discovery.

[garethsb]

A Broadcast Controller uses the DNS-SD advertisement to discover the location of the Network Control API, if it is not statically configured.

[NEOAdvancedTechnology]

Sachin will work offline to discuss optional vs. mandatory

[garethsb]

We could add an initial sentence up-front that says "If Network Control clients are not statically configured with the location of the Network Control API, they should perform discovery according to the procedure below"?

[NEOAdvancedTechnology]

Of note, I can't find anything on Google regarding the use of DNS-SD to find an SDN controller. There are a few theoretical discussions online, but they concentrate on the southbound interface rather than the northbound one. However, there is use of DNS-SD to find sFlow analyzers: https://blog.sflow.com/2010/06/dns-sd.html

[NEOAdvancedTechnology]

Perhaps simplify to: "If Network Control clients are not otherwise configured with the location of the Network Control API, they should perform discovery according to the procedure below".

[garethsb]

Works for me.

[garethsb]

Done. Ready for review again.

[garethsb]

Summarising:

• Broadcast Controller needs to know the Network Controller somehow!
• To avoid manual configuration, this PR proposes using the common NMOS approach of DNS-SD.
  • To satisfy backwards compatibility (for what that's worth given how many deployed IS-06 implementations there are), this is made optional, allowing for alternative configuration mechanisms in the BC
• An alternative to DNS-SD would be to extend the IS-09 System API to add this.

[garethsb]

Note, also resolves #6 by adopting api_proto and api_auth, to follow practice of other NMOS specs.

[NEOAdvancedTechnology]

Sounds good!