Re: [netmod] Joint WGLC on "semver" and "module-versioning" drafts

[Martin Björklund <[e-mail]>]

Hi,

But the two drafts go way beyond fixing the problem your three
examples illustrate.  If the goal is to indicate that non-backwards
compatible changes have occured, a single new extension statement
could solve that.  (As I probably have stated before, personally I
don't think this is necessary).

Apart from the updates to RFC 7950 section 11, I am mostly concerned
about the additional complexity the "pluggable" revision-label scheme
brings.




/martin




"Rob Wilton \(rwilton\)" <rwilton=[e-mail]> wrote:
> I'm wondering whether we are in the realm of missing the bigger
> picture here, or perfection being the enemy of good enough.
> 
> My first example:
> 
> The sedate WG (https://datatracker.ietf.org/wg/sedate/about/) has
> recently been rechartered to respecify the meaning of the date string
> in a non-backwards compatible way.  Yes, this same date string format
> that is very widely implemented and deployed.  I originally had a
> block on the new charter until it was pointed out that the IETF
> specification was being updated because it was inconsistent with the
> ISO time specification and inconsistent with how the date string was
> actually being used by implementations.  I.e., the specification is
> being updated to reflect reality.  I.e., fixing the specification in a
> non-backwards compatible way ends up being pragmatically the right
> thing to do (and this is entirely allowed by the IETF process).
> 
> Ideally, the date-and-time typedef in YANG would also be updated to
> match the update to the definition in RFC 3339 by SEDATE.  But this is
> clearly not compliant with section 11 of RFC 7950 (because the value
> space of allowed values is being narrowed).  The only available choice
> would be to define a new date-and-time-2 typedef which modules could
> then update to.  Of course, you cannot update the existing leaves to
> use the new date-and-time-2 typedef because that also violates section
> 11.  So, you end up with two datetime leaves everywhere the
> date-and-time typedef is used, hopefully with one deprecated (and at
> some point, obsoleted).  Of course, defining the new datetime version
> leaves could also break any loosely related modules that may have
> xpath expressions dependent on that date-time leaf (that the updating
> module author may not even know about) which would need to be updated
> to depend on either of the leaves.  I also don't think that RFC 7950
> is clear about whether deprecated leaves must be implemented by all
> implementations or not, so realistically clients will need to handle
> setting either (or perhaps in some cases, both) of the datetime
> leaves, depending on implementation, probably with a different mix
> across modules (in vast stages of being updated).  What happens if
> some instances of those datetime leaves are mandatory configuration
> and become obsolete?  Is a client required to set it or not, the
> pragmatic answer being that again RFC 7950 is unclear and again this
> will likely be implementation dependent.  What about if some of those
> datetime leaves are list keys?  I believe that the only solution that
> RFC 7950 allows for would be to duplicate the list, deprecating the
> old one, again requiring updates to all augmenting modules, and
> corresponding impact and churn on clients and servers.
> 
> I suspect that OpenConfig may also have a date-and-time typedef.  I
> can be certain about how they would handle this same issue - they will
> just update the definition.  Some clients/servers may have minor
> impacts when they update to a new version of the model, but the impact
> and effort required is minimal, and I think several orders of
> magnitude less then the potential resultant churn than would happen by
> strictly following the RFC 7950 section 11 rules.
> 
> Some may argue that I'm not being pragmatic, and that this could just
> be handled as a bugfix, changing the existing type.  This is one of
> the key things that the YANG versioning is trying to accomplish and
> allow.  It isn't aiming to say that module designers have carte blanch
> to change modules in non-backwards compatible ways.  Instead, it is
> saying that in some cases, the pragmatic solution is to knowingly
> break the RFC 7950 rules and make a breaking change because that
> causes less impact.  Further, a key aim of the versioning work is that
> it is much better to be explicit that a breaking change has occurred
> such that a client can easily be warned of that change and take any
> mitigation necessary - which in the datetime instance above, is quite
> possibly that no mitigation is required at all.
> 
> Finally, I will note that rfc-6691-bis contains a change to the
> datetime definition that is not backwards compatible with the existing
> definition because the semantics of the leaf are being redefined.
> 
> 
> A somewhat similar second example:
> 
> The YANGs IP address type handling of zone information is very similar
> to my first issue, where OpenConfig came to the pragmatic conclusion
> that (in their models) 100% of the use cases of IP addresses only use
> the numeric form without zone identifiers, and hence when someone sees
> the typedef ip_address, this is what they are thinking of, so they
> just pragmatically updated their definition of ip_address type.
> 
> Somewhat related to this, I will note that rfc-6691-bis contains a
> change to the ipv4-address and ipv6-address regex definition that is
> not backwards compatible with the existing definition (it narrows the
> valuespace for zone-ids restricting it to ASCII letters and digits
> whereas previously it allowed for any language letters or digit
> characters).  I believe that this change is not strictly compatible
> with RFC 7950 section 11, but I still think that this is the
> pragmatically right change to make without introducing a new set of IP
> address types, despite the fact that it could hypothetically break
> some clients/servers, and we have no way of knowing in advance if that
> will happen.
> 
> 
> A third consideration:
> 
> Yesterday, Jeff and Mahesh presented in a NETMOD interim on their
> learnings from trying to write the IETF BGP model.  One of their
> outcomes is that they think that some of the other models recently
> standardized by the IETF don’t interoperate well with the BGP model
> and will need to be revised.  I've no idea whether those changes can
> all be made cleanly in a backwards compatible way, but I suspect not.
> Hence, my concern here is that the IETF doesn't really have a great
> path to getting a viable set of YANG models that work together,
> because just publishing modules working in isolation doesn't solve the
> industry problems.
> 
> Because lots of the IETF YANG models have been written without a lot
> of implementation experience (chicken and egg problem), often my
> people who know the protocols but are not experts on YANG, means that
> we can be sure that there are likely to be many bugs and flaws in the
> YANG module RFCs that will need to be fixed or improved.  I would
> expect that some of these cannot be pragmatically fixed in a backwards
> compatible way.
> 
> ---
> 
> My interpretation of the recent last call review comments is the
> suggestion that we pivot to find a fundamentally different solution or
> approach to solving this problem as an RFC7950bis.  I believe that
> would be a mistake.
> 
> In summary, a group of participants have been diligently working on
> this problem space for 5+ years.
> 
> We have had a design team working on this area, and that solution was
> then adopted by the WG.  The authors and interested individuals
> working on this area has presented updated drafts and updates to the
> work at every IETF meeting for the last, 4+ years.  Feedback at the
> various stages/reviews/etc has always been considered, the authors
> meetings have always been open, and I don't believe that the solution
> drafts being taken to WG LC are architecturally significantly
> different from the direction agreed during WG adoption of the
> documents, although I do think that the documents are much improved
> based on the feedback received.
> 
> I also appreciate that Juergen has always publicly stated that this
> work should be done as an update to the YANG language, but my
> recollection was that he was in the rough on this issue, i.e., during
> WG adoption, and since, at least until this IETF WG LC review.
> 
> Hence, my concern, as an AD, is that if, after 5 years, the WG now
> wants to take a fundamentally different path to standardizing this
> work then I have concerns that the NETMOD WG isn't really functioning
> properly and cohesively as a WG, and I'm very concerned that we won't
> find any viable way forward for this work.  I doubt that it will be
> possible to get any quick consensus by opening up RFC 7950.  We may
> also find that the individuals who have invested a significant amount
> of time and effort on this work don't have the desire or energy to
> start from scratch again, when they have a solution that is good
> enough for their needs.
> 
> If I understand correctly, the fundamental objection to the module
> versioning draft is around the updates to section 11 of RFC 7950,
> which effectively state that changes MUST be backwards compatible,
> whereas this draft states SHOULD be backwards compatible, without a
> change to the YANG version number.  Is that correct?
> 
> If the existing deployment and evolution of YANG modules among
> vendors, OpenConfig, IETF, and other SDOs strictly followed the rules
> in RFC 7950 then I would probably agree that an update from YANG 1.1
> to YANG 1.2 is needed.  But I think that the reality is that tools
> must handle non-backwards compatible changes frequently happening in
> YANG 1.0 (OpenConfig) and YANG 1.1 YANG modules anyway.  I.e., I don't
> believe that the "YANG 1.1 no breaking change contract" is being
> widely honoured anyway, and instead is being treated as a goal or
> aspiration.  What these documents attempt to do is to move YANG module
> evolution from what we currently have now where clients don't have any
> way of really knowing how a module has evolved and whether they are
> impacted to one that they do, and as part of that process they are
> aiming to update the YANG versioning rules to better reflect how is it
> being deployed and used.
> 
> Hence, as am author, I still of the opinion that the best pragmatic
> path forward is to try and get these documents to a shape where they
> achieve rough consensus and are acceptable to the WG to be published
> now, in the short term, as a good enough solution.  After that point,
> then I think that it would be great for some folks to form an idea on
> a what YANG 1.2/2.0 could look like, but I think that coupling these
> goals together would be a mistake.
> 
> Regards,
> Rob
> 
> // Who doesn't really know which hat he is wearing for this comment,
> but is only trying to do the right thing for the wider industry ...
> 
> 
> > -----Original Message-----
> > From: netmod <[e-mail]> On Behalf Of Jürgen Schönwälder
> > Sent: 06 June 2023 06:07
> > To: Martin Björklund <[e-mail]>
> > Cc: [e-mail]
> > Subject: Re: [netmod] Joint WGLC on "semver" and "module-versioning"
> > drafts
> > 
> > On Mon, Jun 05, 2023 at 10:32:51PM +0200, Martin Björklund wrote:
> > > >
> > > > If the goal is to produce YANG 1.2 which (i) integrates semantic
> > > > versioning into YANG and (ii) fixes known bugs in YANG 1.1 and (iii)
> > > > does not add any other new features, then having agreement on such a
> > > > statement will help to steer the process.
> > >
> > > I hope that (i) doesn't happen.  I think it is the proposed changes in
> > > draft-ietf-netmod-yang-module-versioning that require a new YANG
> > > version.  If this new YANG version allows for other versioning schemes
> > > than revision-date, then we can keep the modified semver scheme
> > > outside the core document.
> > >
> > 
> > I consider the module update rules a part of a versioning model. The
> > current update rules were written to support the current versioning
> > model. If we want to support multiple versioning models, then we have
> > to refactor the update rules out of the YANG language specification
> > into separate versioning specifications, i.e., traditional YANG
> > versioning and the new semver versioning. There are some language
> > mechanisms (like the import statement), that have to be flexible
> > enough to support multiple versioning schemes.
> > 
> > Is it worth factoring the versioning model out of the language? I
> > guess the opinions vary widely on this, depending on the dynamics of
> > the software environment people are working in.
> > 
> > /js
> > 
> > --
> > Jürgen Schönwälder              Constructor University Bremen gGmbH
> > Phone: [phone]         Campus Ring 1 | 28759 Bremen | Germany
> > Fax:   [phone]         <https://constructor.university/>
> > 
> > _______________________________________________
> > netmod mailing list
> > [e-mail]
> > https://www.ietf.org/mailman/listinfo/netmod
> _______________________________________________
> netmod mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/netmod