Hi Martin,
If a "description" of a leaf (without a default statement) changes from this:
"the absence of this leaf causes the protocol to stay administratively
down"
to this:
"the absence of this leaf causes the protocol to go administratively up"
(with no other changes in the YANG) then IMO it *is* an NBC change. The
behavior described in a "description" field is considered part of the model/API
(I've seen many references & examples of this over the years in NETMOD/NETCONF
discussions).
Maybe it becomes more subtle if that behavior change isn't documented in the
"description" statement (I'd argue it is still NBC if the server changes that
behavior and they should be publishing a new revision of the YANG model/API),
but I was proposing that it should in this case.
Jason
> -----Original Message-----
> From: Martin Björklund <[email protected]>
> Sent: Friday, April 14, 2023 4:39 AM
> To: Jason Sterne (Nokia) <[email protected]>
> Cc: [email protected]; [email protected]
> Subject: Re: [netmod] Unknown bits - backwards compatibility
>
>
> CAUTION: This is an external email. Please be very careful when clicking
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>
>
> Hi,
>
> I am quite confused after reading this thread, so I had to go back to
> this first message:
>
> "Jason Sterne (Nokia)" <[email protected]> wrote:
> > Hi Jeff,
> >
> > One topic that came up during the IETF 116 NETMOD meeting was
> > backwards compatibility.
> >
> > >From what I understand, a leaf (e.g. unknown-flags) that uses the
> > >unknown-bits typedef would never change its definition in YANG. It
> > >would always be defined as unknown-bits with all 64 bit definitions
> > >even as more and more bits become "known". *But* the system would
> > >suddenly stop reporting bit-0, then bit-1 in that unknown-flags leaf
> > >as those bits become known.
> >
> > Strictly speaking, that should probably be considered an NBC change
>
> Nothing has changed in the data model, so there is no way to mark the
> _data model_ as NBC.
>
> The server follows the data model, and reports which bits it doesn't
> understand. With software updates, this may change over time. This
> is simply the semantics of this state leaf.
>
>
> /martin
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