Clarify canonical representation of typedefs

[rgwilton]

Some typedefs (and possibly some leaves) (e.g. ipv6-prefix) define their own canonical representation of the value space that overrides the canonical representation defined in the YANG built in types (RFC 7950 section 9.1).

I think that the next revision of the YANG draft should be clarified that:

• typedefs and leaves are allowed to define their own refined canonical representation of values.
• servers MUST convert values into the typedef's canonical representation on configuration/rpc/action, internally handle in the canonical representation, and always return the canonical representation.

I also think that we should consider whether YANG would benefit from a new keyword to mark typedefs or leaves that override the canonical representation of the base type, as an aid to compilers.

Further details/discussions on NETMOD "6021 ipv4-prefix" thread.

[llhotka]

Canonical representation should IMO be used only for types (built-in or derived), not individual leaves.

Instead of introducing a new keyword just as a label, we should maybe consider specifying the canonical format in a machine readable way, e.g. as an expression that transforms any legal value of the type into the canonical format.

[samans]

The use of strings as keys is troublesome when the format provided is not precise. An example is the mac-address typedef that defines a format string that supports both upper and lower case letters. In order for the key to match, they need to be identical. The Canonical representation is described in the description text, Providing a substatement that provides the Canonical representation in a machine readable way could provide a way to convert the value into canonical form before comparing keys.

[abierman]

Support these changes, but also need machine-readable syntax (as much as possible) Canonical format is complicated in servers and clients. Not sure if any existing syntax can help. This is like DISPLAY-HINT for a leaf.

complexity: high, bc: medium, importance: medium

[kwatsen]

This seems important.

Postel's law applies. Liberal in what is received, conservative in what is sent.