[netmod] draft-ietf-netmod-yang-module-versioning-05: NBC Changes

[Andy Bierman <[e-mail]>]

Hi,

The draft-05 version has expired and is no longer on the NETMOD WG WEB page.

Sec 3.1 includes this text:

   Where pragmatic, updates to YANG modules SHOULD be backwards-
   compatible, following the definition in Section 3.1.1
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-yang-module-versioning#section-3.1.1>.



This is a significant departure from RFC 7950.

There are normal "lifecycle" changes that can occur to YANG-based APIs
that are currently classified as NBC changes in 7950, sec 11.
They are often related to the release train. Maybe this concept is
common enough to standardize.

For example, we often introduce a leaf with a default of 'disabled'
because we mandate that ALL new features or behavior changes MUST
require the client to opt-in.  But after a year or two, we may change that
default to 'enabled' in a new release train.  Then if the feature is
deprecated
 at the end of the lifecycle, the default may get changed back to 'disabled'
in a new release train.  BC compatibility is ALWAYS maintained within
a release train.

Given that RFCs take so long to publish, it seems unlikely that
we would ever be in a position to make a "real" NBC change
in the middle of a release train, such that all of the client code
already deployed will break immediately when the upgrade is done.
We can NEVER just remove the old version.

It is hard to imagine a situation where breaking real deployments
is a better option than introducing a new identifier, so old clients
and new clients can coexist.


Andy