Re: [netmod] Interplay between YANG network instances and schema mount

[Martin Bjorklund <[e-mail]>]

Robert Wilton <[e-mail]> wrote:
> Hi,
> 
> The network-instances draft (draft-ietf-rtgwg-ni-model) uses YANG
> mount to allow relevant parts of the YANG schema to be made
> available under a particular network instance (e.g. as per the
> diagram on section 3.2), reproduced inline here:
> 
>       +--rw yanglib:modules-state           [RFC7895]
>       +--rw if:interfaces                   [RFC7223]
>       |  +--rw bind-network-instance-name="green" string
>       +--rw network-instances
>          +--rw network-instance* [name]
>             +--rw name="green"    string
>             +--rw type?                           identityref
>             +--rw enabled=true                    boolean
>             +--rw description="The Green VRF"     string
>             +--rw network-instance-policy
>             |  ... (RT=1000:1, RD=1.2.3.4)
>             +--rw root?                           yang-schema-mount
>                +--rw yanglib:modules-state  [RFC7895]
>                +--rw if:intefaces           [RFC7223]
>                +--rw mm:network-services
>                +--rw nn:oam-protocols
>                +--rw oo:routing
>                +--rw pp:mpls
> 
> 
> My assumption is that the mounted YANG modules are just providing
> alternative paths in the schema for the same underlying data nodes
> that are also available without going via the schema-mount path.

Schema mount in itself does not specify (by design) anything about the
underlying instrumentation of mounted data models.

In the case of a network orchestrator that uses schema mount for
network devices, the mounted paths are obviously not providing
alternative paths.

But I think it is fine if some usage of schema mount wants this
semantics.  It needs to be defined in the module that uses schema
mount.

> I
> further presume that datanodes could be read/written in both places
> (of course subject to NACM), and that any changes to data nodes made
> in one place must be immediately reflected in both places.

> 
> To give a concrete example, assuming that "eth0" was bound to
> network-instance "foo" then:

>  network-instances/network-instance[name="foo"]/root/interfaces/interface[name="eth0"]
>  would be pointing to the same actual data node as
>  /interfaces/interface[name="eth0"].
> 
> Is my interpretation of how schema mount is anticipated to work in
> this scenario correct?
> 
> If my understanding is correct then this would seem to imply:
>  - semantic validation of the the datatree can be performed by
>    ignoring schema mounted nodes altogether. 
>  - leafrefs logically exist independently of the schema mounted nodes.
>  - leafrefs viewed under a schema mount can, in some cases, be
>    simplified to look like the reference is local within the mounted
>    schema tree, but in the case that I'm considering here, it isn't
>    obvious to me that all such leafrefs must necessarily never
>    reference nodes outside of this subtree.

The last bullet might be a problem...


/martin