[core] resource-directory: generic ins= and suitability for DNS-SD

[Christian Amsüss]

Hello CoRE WG,

in the last resource-directory meeting, one of the longest single
discussion items was how to proceed with the ins= attribute. The
question in short is whether a simpler description like

The resource instance attribute "ins" is a textual parameter that
can be used to distinguish between resources of equal resource
types within an endpoint or a domain.

For example, values can be texts like "Ceiling light, Room 3", "Slot
2", short IDs like "AF39", but also UUIDs or iNumbers.

(note that the "unique across the domain" requirement is gone)

would still work for creating RD <-> DNS-SD gateways.

On the missing uniqueness requirement I'd like to point out that unless
the RD is sychronized with DNS-SD (ie. it only responds to registrations
when it has complted DNS-SD entry), the gateway would need to have a
fall-back plan for collisions anyway: The RD might not even have
enforced ins uniqueness, or a client could have not shipped an ins= with
an exp link at all.

Can DNS service discovery based on a resource-directory work with such a
laxer ins attribute, and if not, how would you expect an RD to handle
duplicates of ins values?

Best regards
Christian