Re: [netmod] handling module incompatibility => handling module transition

Benoit Claise <> Sun, 08 October 2017 14:58 UTC

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To: Lou Berger <>, NetMod WG <>
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From: Benoit Claise <>
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Date: Sun, 08 Oct 2017 16:58:32 +0200
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Subject: Re: [netmod] handling module incompatibility => handling module transition
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Dear all,

Focusing on draft-wu-l3sm-rfc8049bis, the big problem is: RFC8049 is 
broken. The small problem is: trying to maintain backward compatibility.
draft-wu-l3sm-rfc8049bis has rightly focused on the big problem.

Let me extend the scope of this email thread from "handling module 
incompatibility" to "handling module incompatibility and NMDA transition".
As I mentioned in the past (see " comparison of two YANG 
modules" email in NETMOD), I believe the IETF will have to change its 
way of working in terms of backward compatibility. See also the email 
"ietf-routing or ietf-routing-2? module naming convention for NMDA 
transition. Re: [netmod] upcoming adoptions" in NETMOD.

However, we will have to tackle this discussion one day or the other:
- we need _an automatic way_ to make the link between the YANG module in 
RFC8049 and the YANG module in draft-wu-l3sm-rfc8049bis, or any non 
backward compatible YANG modules.
- we need _an automatic way_ to make the link between the YANG module in 
RFC8022 and the YANG module in draft-acee-netmod-rfc8022bis, or any new 
YANG module names used for NMDA transition.
Note: actually, we face two different problems. draft-wu-l3sm-rfc8049bis 
might be declared backward incompatible with RFC8049, while RFC8022bis 
is backward compatible with RFC8022. The RFC8022bis went for a new YANG 
module name ietf-routing-2 to avoid to document the -state tree (as 
deprecated), based on the argument that ietf-routing is not yet implemented.

Which solutions do we have from here?
#1. We keep the same module name and express that the YANG module in 
draft-wu-l3sm-rfc8049bis is not backward compatible with the RFC8049 
one. This is the openconfig way. See draft-clacla-netmod-model-catalog 
(and draft-openconfig-netmod-model-catalog before)

       // extension statements
          extension openconfig-version {
            argument "semver" {
              yin-element false;
              "The OpenConfig version number for the module. This is
              expressed as a semantic version number of the form:
                * x corresponds to the major version,
                * y corresponds to a minor version,
                * z corresponds to a patch version.
              This version corresponds to the model file within which it is
              defined, and does not cover the whole set of OpenConfig models.
              Where several modules are used to build up a single block of
              functionality, the same module version is specified across each
              file that makes up the module.

              A major version number of 0 indicates that this model is still
              in development (whether within OpenConfig or with industry
              partners), and is potentially subject to change.

              Following a release of major version 1, all modules will
              increment major revision number where backwards incompatible
              changes to the model are made.

              The minor version is changed when features are added to the
              model that do not impact current clients use of the model.

              The patch-level version is incremented when non-feature changes
              (such as bugfixes or clarifications to human-readable
              descriptions that do not impact model functionality) are made
              that maintain backwards compatibility.

              The version number is stored in the module meta-data.";

Similarly, we always keep the same YANG module name in case of NMDA 
transition. So ietf-routing-2 should be changed back to ietf-routing.
The email thread "[Rtg-dt-yang-arch] ietf-routing or ietf-routing-2? 
module naming convention for NMDA transition. Re: [netmod] upcoming 
adoptions" seems to go in that direction.

#2. Or we have a different module name, let's say ietf-l3vpn-svc-2 or 
ietf-routing-2 but then, how do we make the link with the previous module?
Then ...  What? We create extension that will link the 
draft-wu-l3sm-rfc8049bis YANG module to the RFC8049 YANG module? Same 
principle as #1, but just more complex.
Or we have a new YANG keyword (this implies YANG 2.0)

    <CODE BEGINS>file"ietf-l3vpn-svc@2017-09-14.yang"
    module ietf-l3vpn-svc-2 {
      yang-version 1.1;
      namespace "urn:ietf:params:xml:ns:yang:ietf-l3vpn-svc";
      prefix l3vpn-svc;
      *_obsolete|update _*ietf-l3vpn-svc@2017-01-2

And whose responsibility is this to warn/push all authors of 
ietf-routing YANG modules to move to ietf-routing-2? (*)

The following are non solution IMO:
- Going from the draft-wu-l3sm-rfc8049bis YANG _module _to the 
draft-wu-l3sm-rfc8049bis _document _to lookup the IETF "obsolete" flag 
in order to understand that the RFC8049 YANG module is obsolete is not 
an automatic solution.
- Using the might be a solution as we track the derived 
semantic, but this is just an offline trick. This is not what I call 
"automatic way"
- Using the YANG module description field might be interesting, but 
again this is not an "automatic way":

       "This YANG module defines a generic service configuration
        model for Layer 3 VPNs. This model is common across all
        vendor implementations. This obsoletes the RFC8049 YANG
        module, ietf-l3vpn-svc@2017-01-2";
      revision 2017-09-14 {
        "First revision ofRFC8049 <>.";
        "RFC xxxx: YANG Data Model for L3VPN Service Delivery";

In conclusion, I believe openconfig got this right and that solution #1 
is the solution to go ... while waiting for a new YANG keyword in YANG 2.0

(*) If you want to change the module from ietf-routing to 
ietf-routing-2, then you should follow with all authors of dependent 
modules to make sure they transition to ietf-routing-2
In the, because I needed the information as OPS AD, we 
created a small script to get that authors list for IETF drafts. For the 
ietf-routing, this produces the following
     "output": {
         "author-email": [

Fortunately, we only deal with IETF dependent YANG modules in case of 
the ietf-routing. That's an easier case.
If we would change ietf-interfaces to ietf-interfaces-2, we would have 
an cross SDO issue ... Btw, there are no automatic ways to extract the 
authors of YANG modules from different SDOs: it's only a metadata that 
that the different SDOs should insert in the yangcatalog. So we would 
have to rely on liaisons or direct emails, assuming we know the authors. 
Time consuming, believe me.

Regards, Benoit
> Hi,
>      As part of the my Routing Directorate review of
> draft-wu-l3sm-rfc8049bis I noted that there were several incompatible
> changes being made to the ietf-l3vpn-svc module without changing the
> name.  I raised this with the YANG doctors and others involved with the
> draft and it surfaced some topics which really should be discussed here
> in NetMod.
> The background (as explained off-list by others, so I hope I have it
> right)  is that one of the YANG Doctors noted that RFC8049 was broken
> and could not be implemented as defined, and therefore a fix was
> needed.  L3SM has concluded so the fix is in the individual draft
> draft-wu-l3sm-rfc8049bis.  Since the rfc8049 version of ietf-l3vpn-svc
> module could not be implemented, the feeling by the YANG Dr was that
> even though the new module is incompatible with the original definition
> the module the rule defined in Section 11 of YANG 1.1 (or section 10 of
> RFC 6020) didn't have to be followed and the same name could be used.
> In the subsequent discussion with the YANG Drs., the general discussion
> was heading down the path of using a new module name, and thereby not
> violating YANG module update rules.  This lead us back to the a similar
> discussion we've been having in the context of 8022bis: how best to
> indicate that a whole module is being obsoleted.  RFCs do this by adding
> 'metadata' to the headers, e.g., "Obsoletes: 8049", but this doesn't
> help YANG tooling.  For 8022, we have one approach - publishing an
> updated rev of the original module marking all nodes as deprecated - but
> that doesn't really make sense for rfc8049bis.
> So the discussion for the WG is:
> How do we handle incompatible module changes, notably when one module
> 'obsoletes' another module --  from both the process and tooling
> perspectives?
> I know Benoit plans to bring in some thoughts/proposals, perhaps there
> are others.
> Cheers,
> Lou
> (as contributor/reviewer)
> .