Re: [netmod] Questions about how to assign default values with YANG

On Tue, Mar 9, 2021 at 8:46 AM Ladislav Lhotka <ladislav.lhotka@nic.cz>
wrote:

> Italo Busi <Italo.Busi@huawei.com> writes:
>
> > Hi Juergen,
> >
> > Thanks again for your clear explanation on this topic
> >
> > I have found a similar but slightly different issue. In this case, a
> YANG default statement exists in the base module but the intention with the
> augmentation is to "overwrite" the default value on the basis of another
> attribute, defined in the module which augments the base module.
> >
> > For example, I am wondering whether such a code is valid:
>
> Yes, this is valid, I'd just suggest:
>
>
I do not agree.
I do not see how the description-stmt for /foo can change the default leaf
processing for /bar

> - remove the default statement for "foo", as it may be confusing to both
> humans and tools
>

sec 7.3.4:

   If the base type has a default value and the new derived type does
   not specify a new default value, the base type's default value is
   also the default value of the new derived type.

sec 7.6.1

   The default value of a leaf is the value that the server uses if the
   leaf does not exist in the data tree.  The usage of the default value
   depends on the leaf's closest ancestor node in the schema tree that
   is not a non-presence container (see Section 7.5.1
<https://tools.ietf.org/html/rfc7950#section-7.5.1>):

   o  If no such ancestor exists in the schema tree, the default value
      MUST be used.

   o  Otherwise, if this ancestor is a case node, the default value MUST
      be used if any node from the case exists in the data tree or the
      case node is the choice's default case, and if no nodes from any
      other case exist in the data tree.

   o  Otherwise, the default value MUST be used if the ancestor node
      exists in the data tree.

> - specify the default (both cases) in the description of "foo"
>
> A similar example is in the module ietf-ipv6-router-advertisements, e.g.
> leaf "min-rtr-adv-interval":
>
> https://www.rfc-editor.org/rfc/rfc8349.html#section-9.1
>
> Lada
>

Andy

>
> >
> > module example-base {
> >   container example {
> >     leaf foo {
> >       type uint8;
> >       default 0;
> >     }
> >   }
> > }
> >
> > module example-augment {
> >   import example {
> >     prefix ex;
> >   }
> >
> >   augment "ex:example" {
> >     leaf bar {
> >       type empty;
> >       description
> >         "When present, the default value for foo is 10.";
> >     }
> >   }
> > }
> >
> >
> > In this case, when the leaf foo is not configured but the leaf bar is
> present, the value of foo in the operational datastore should be 10 (rather
> than 0).
> >
> > In this case, I think that it would be better/cleaner if the origin is
> marked as system.
> >
> > Maybe a better YANG description for bar could be: "When present, the
> system overrides the default value of foo to 10."
> >
> > What is your and/or WG opinion?
> >
> > Thanks again
> >
> > Italo
> >
> >> -----Original Message-----
> >> From: Juergen Schoenwaelder [mailto:
> j.schoenwaelder@jacobs-university.de]
> >> Sent: mercoledì 20 gennaio 2021 17:05
> >> To: Italo Busi <Italo.Busi@huawei.com>
> >> Cc: 'netmod@ietf.org' <netmod@ietf.org>
> >> Subject: Re: [netmod] Questions about how to assign default values with
> >> YANG
> >>
> >> On Wed, Jan 20, 2021 at 02:41:39PM +0000, Italo Busi wrote:
> >> >
> >> > What about the case the leaf is not conditional (but still mandatory
> false
> >> since a YANG default statement is defined)?
> >> >
> >> > May the server still decide not to use/implement this leaf in the
> operational
> >> datastore?
> >> >
> >> > For example, in appendix C.1 of RFC8342, auto-negotiation is enabled
> by
> >> default.
> >> > What should be the behavior of a system which does not implement auto-
> >> negotiation?
> >> > Return the value false or no value (in the operational datastore)?
> >> >
> >>
> >> Here are some of the rules I personally like:
> >>
> >>  - <operational> is the ground truth about what a system has and does
> >>  - do not implement leafs that do not apply
> >>
> >> Hence, interfaces supporting auto-negotiation have either auto-
> >> negotiation/enabled = true or auto-negotiation/enabled = false in
> >> <operational>. And interfaces not supporting auto-negotiation have
> nothing
> >> to report about auto-negotiation. Yes, I do not want to see auto-
> >> negotiation/enabled = false on a loopback interface.
> >>
> >> My historic Ethernet interface from the last century would also not
> report
> >> auto-negotiation/enabled in <operational>. You may hit applications
> that love
> >> to have auto-negotiation/enabled available on all Ethernet interfaces
> and then
> >> you end in a debate where the application developers tell you that no
> >> information in <operational> may have many reasons (instrumentation not
> >> implemented, access control rules, whatever and by reporting
> enabled=false
> >> you do them a favor) but the true answer in such a debate is often that
> >> modeling things as a boolean is simplistic since there are often more
> than
> >> exactly two states (in this case, enabled, disabled, failed,
> not-available, ...).
> >> So you settle on blaming the model writer. ;-)
> >>
> >> /js
> >>
> >> --
> >> Juergen Schoenwaelder           Jacobs University Bremen gGmbH
> >> Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
> >> Fax:   +49 421 200 3103         <https://www.jacobs-university.de/>
> >
> > _______________________________________________
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> > https://www.ietf.org/mailman/listinfo/netmod
>
> --
> Ladislav Lhotka
> Head, CZ.NIC Labs
> PGP Key ID: 0xB8F92B08A9F76C67
>
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