[netmod] Interplay between YANG network instances and schema mount

[Robert Wilton <[e-mail]>]

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. 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.

Thanks,
Rob