Re: [netmod] Question about schema-mount

Martin Bjorklund <mbj@tail-f.com> Thu, 28 November 2019 08:56 UTC

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Date: Thu, 28 Nov 2019 09:56:05 +0100 (CET)
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Subject: Re: [netmod] Question about schema-mount
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Hi,


"Bogaert, Bart (Nokia - BE/Antwerp)" <bart.bogaert@nokia.com> wrote:
> Hi Martin,
> 
> -----Original Message-----
> From: Martin Bjorklund <mbj@tail-f.com> 
> Sent: Thursday, November 28, 2019 9:28 AM
> To: Bogaert, Bart (Nokia - BE/Antwerp) <bart.bogaert@nokia.com>
> Cc: netmod@ietf.org
> Subject: Re: [netmod] Question about schema-mount
> 
> Hi,
> 
> "Bogaert, Bart (Nokia - BE/Antwerp)" <bart.bogaert@nokia.com> wrote:
> > Hi,
> > 
> > We're trying to figure out whether it is possible to define a module 
> > in the parent schema which would use a node being a leafref to a node 
> > in a model under a mount point.
> 
> In order for this to work, the leafref target would have to be present
> at compile time / design time.  RFC 8528 defines Design time,
> Implementation time, and Run time mounts, and says: "Design-time
> mounts are outside the scope of this document" (see section 1).
> 
> Another alternative would be some new kind of "leafref-like" construct
> that supported this.
> 
> What you can do though is to ensure the leaf in the parent module is
> of the same type as the mounted node you want to refer to, and explain
> in text what the semantics is.
> 
> [Bogaert, Bart] Thanks for this feedback.  The use case we're looking
> into is the following:
> [Bogaert, Bart] A forwarder contains references to interfaces which
> are defined as leafref with path /interfaces/interface/name.  In the
> scope of this question the path would be
> /logical-network/elements/logical-network-element/interfaces/interface/name.
> Question is: will this work with leafref?

No, since a YANG compiler that sees this leafref will fail to find the
target and thus throw an error.

[side note: a single leafref wouldn't work anyway, since you would the
name of both the lne and the name of the interface]

> What will the [Bogaert,
> Bart] NETCONF server do in such a case, in other word: is it able to
> resolve this leafref or do we have to define this as a some kind of
> string that includes the name of the logical-network-element and the
> name of the interface?

Yes, a single string or two leafs, 'lne-name' and 'if-name'.


/martin


> > I don't seem to find any statement
> > that would prohibit this but RFC 8530, referred to from the schema 
> > mount RFC, uses a leafref to a node in a module which is still in the 
> > list of YANG modules of the parent (and consequently in the YANG 
> > library of the parent).
> > 
> > So, using RFC8530 as example:
> > 
> > Instead of lne:bind-lne-name being a leafref to 
> > /logical-network-elements/logical-network-element/name we would point 
> > to 
> > /logical-network-elements/logical-network-element/interfaces/interface/name.
> > 
> > The interfaces YANG module is also part of the YANG library of the 
> > parent but I'm not so sure whether above construction would work well 
> > as the information related to the mounted YANG modules is in a YANG 
> > library different from the parent's YANG library.
> > 
> > Note that there is also some confusion with the examples in RFC8530:
> > while the bind-lne-name in the YANG module is a leafref as syntax, the
> > examples work with a string as syntax for that same leaf.
> 
> Do you mean the tree diagram?
> 
> [Bogaert, Bart] Correct.
> 
> Best regards, Bart
> 
>    module: ietf-interfaces
>      +--rw interfaces
>         +--rw interface* [name]
>            +--rw name                        string
>            +--rw lne:bind-lne-name?          string
> 
> This looks like a bug to me; it should be a leafref.
> 
> 
> /martin
>