[netmod] NBC changes and warnings

[Andy Bierman <[e-mail]>]

Hi,

Before any significant NBC change can be introduced, a process needs
to be in place to minimize the disruption that the NBC change will cause.

Consider the recent change by GitHub to remove support for SSH passwords
and require keys instead.  They warned people for about 2 years this change
was coming.  Not just on the WEB site, but in the tools. Every time
a user logged in over SSH with a password, a warning was printed.

For those willing to read the warning and act upon it, there was no
service disruption at all when SSH passwords were finally turned off.

When NETCONF was introduced in 2006, everything was new.
The NETCONF error-severity=warning did not even work, and nobody cared.
It still doesn't work.

There are other possibilities besides
"We can never make an NBC change because nobody will read the warning
in the description-stmt anyway."  That's true. So let's fix it in the
protocols
and in the YANG language, so they do get the warning.


Andy