Re: [netmod] adoption poll for yang-versioning-reqs-02

Martin Bjorklund <> Wed, 20 March 2019 07:30 UTC

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Subject: Re: [netmod] adoption poll for yang-versioning-reqs-02
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"Rob Wilton (rwilton)" <> wrote:
> Hi Martin,
> Thanks for the review and comments.
> A couple of points:
> 1) Lots of models outside those published in SDOs are already not
> following the RFC 7950 revision rules.

Right, and I think that is ok.  If vendors want to break backwards
compatibility it is up to them.  With the current rules you can have
tool that detect this and flags it.  You can then fix the problem or
release the new NBC module anyway, but you have been warned.
Customers will accept this or not.

> I think that it is better to
> have a versioning scheme that reflects how YANG models are actually
> evolving rather than have all vendor and OC YANG modules either just
> ignoring the rules, or using clever tricks that strictly conform with
> the rules but go against the spirit of them (e.g. just publish an
> entirely new set of YANG modules for each release).  Also noting that
> having a scheme that allows non-backwards-compatible changes does not
> require that everyone uses them - IETF could continue to always
> publish backwards compatible modules.  The obvious alternative here is
> that each vendor comes up with their own versioning extension and
> ignores the RFC 7950 section 11 rules anyway, but I'm not sure how
> that really helps client<->server interop.

The client<->server interop will not magically work better if we allow
NBC changes.

> 2) I don't understand how the RFC 7950 approach of "deprecate a buggy
> node, and replace with a working node" really works in practice,
> particularly for configuration data nodes where you have two clients
> interacting with a server, one interacting with the old path, and
> another using the new path.  Perhaps there is a robust scheme that
> works in all cases, but it isn't obvious to me.  Historically, for CLI
> we just translate the CLI from old to new format and then return the
> new format when the running config is requested.  But that will still
> break an old client that doesn't understand how to read the new CLI,
> even if the server supports them writing via the old CLI.
> Even if there is a workable solution for this simple case, I suspect
> that there are many slightly more complicated cases that don't work
> (e.g. rekeying a list, changing defaults, incompatible types).

I fully agree.  This is difficult in the general case.  In the worst
case you'll have to give up on backwards compatibility and only
implement the new version of the module (I believe this is possible
both with the current versioning rules, and with the proposed rules).

> In short, I don't agree with the premise that the current YANG
> versioning schema using revision dates is working just fine, and no
> changes are needed.

But this is not what the draft is about!  There is nothing in the
draft that talks about problems introduced by using the revision


> Thanks,
> Rob
> -----Original Message-----
> From: netmod <> On Behalf Of Martin Bjorklund
> Sent: 19 March 2019 15:12
> To:
> Cc:
> Subject: Re: [netmod] adoption poll for yang-versioning-reqs-02
> Hi,
> I have read this document, and I do not think it should be adopted.
> I object to the idea that we should allow non-backwards-compatible
> changes to published YANG modules.
> The draft motivates this idea with:
>    we must recognize that many YANG
>    modules are actually generated YANG modules (for example, from
>    internal databases)
> I do not agree that we should change what we allow in published
> modules b/c of this.
> It also motivates this idea with:
>    The points made above lead to the logical conclusion that the
>    standardized YANG modules have to be perfect on day one (at least the
>    structure and meaning), which in turn might explain why IETF YANG
>    modules take so long to standardize.
> I disagree with this.  First of all, we have already published
> revision two of several YANG modules (ietf-inet-types, ietf-yang-type,
> ietf-interfaces, ietf-ip, ietf-routing, ...), so the statement that
> "standardized YANG modules have to be perfect on day one" is simply
> not true.
> Second, I don't think the upgrade rules are the reason it takes a long
> time to standardize IETF models (I think it has to do with the process
> itself, including the fact that models get reviews from many different
> people with different background.)  [BTW, is it true that drafts with
> YANG models take longer time from wg -00 to published RFC than other
> drafts?]
> This said, I think there are some important points that the draft
> raises, and that I think we should continue to work on; specifically
> 2.3, 2.5, 2.6, 2.7.  But I don't think that these areas require
> changes to the versioning scheme, and I think it is a mistake to
> include these areas in this draft.
> Some comments on section 4, The Problem Statement:
>    o  Any non-backwards-compatible change of a definition requires
>       either a new module name or a new path.  This has been found
>       costly to support in implementations, in particular on the client
>       side.
> Yes I agree there is a cost associated with this.  But I have come
> across vendor modules that make NBC changes w/o introducing a new
> path, and this is also costly to handle.
>    o  Since non-backwards-compatible changes require either a new module
>       name or a new path, such changes will impact other modules that
>       import definitions.  In fact, with the current module versioning
>       scheme other modules have to opt-in in order to use the new
>       version.  This essentially leads to a ripple effect where a non-
>       backwards-compatible change of a core module causes updates on a
>       potentially large number of dependent modules.
> This is by design.  We cannot have a situation where a legal
> modification to a module leads to other modules becoming invalid.
>    o  YANG has a mechanism to mark definitions deprecated but it leaves
>       it open whether implementations are expected to implement
>       deprecated definitions and there is no way (other than trial and
>       error) for a client to find out whether deprecated definitions are
>       supported by a given implementation.
> As I wrote above, I agree that this is a problem that should be
> solved.  But this is not a motivation for changing YANG versioning.
>    o  YANG does not have a robust mechanism to document which data
>       definitions have changed and to provide guidance how
>       implementations should deal with the change.  While it is possible
>       to have this described in general description statements, having
>       these details embedded in general description statements does not
>       make this information accessible to tools.
> This might also be worth exploring, but this is not a motivation for
> changing YANG versioning.
> /martin
> Kent Watsen <> wrote:
> > Seeing as how we all need to read this draft anyways, in preparation
> > for our meeting in Prague, it seems like a good time for this poll.
> > Thusly, this email begins a 1-week adoption poll for:
> > 
> >     
> >
> > <
> > 2>
> > 
> > Please voice your support or objections before March 20.
> > 
> > Note that this draft defines *requirements* and its intended status is
> > "Informational."  I believe that it is good for WGs to formalize
> > requirements, even taking such drafts thru Last Call, in order to
> > ensure consensus on the requirements.  This is the "adoption" call, to
> > ascertain if the WG agrees with that statement; if adopted, a separate
> > "last call" will be issued to ensure to correctness of the draft's
> > content.
> > 
> > Kent (and Lou and Joel)
> > 
> > 
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