Re: [mpls] [netmod] Use of schema mounts for common model

["Tarek Saad (tsaad)" <[e-mail]>]

Thanks Martin, please see inline..


On 2016-04-29, 6:29 AM, "Martin Bjorklund" <[e-mail]<mailto:[e-mail]>> wrote:

"Tarek Saad (tsaad)" <[e-mail]<mailto:[e-mail]>> wrote:
Hi authors/WG,
In draft-ietf-teas-yang-te, we are driving the definition for a
generic TE YANG model that can/may be used (and extended when
necessary) for different data plane technologies (e.g. MPLS, OTN, WDM,
etc.).
Reviewing the schema mount idea presented in
draft-ietf-netmod-schema-mount, we are thinking this proposal is
useful and can facilitate the reuse of the our model in multiple
places in the YANG tree (once per each technology), e.g.:
…/mpls/mount-points/mount-point/module=ietf-te.yang
…/otn/mount-points/mount-point/module=ietf-te.yang

Schema mount is probably not the right solution to your problem.  I
think a better solution in your case is to define groupings.
Groupings are designed to be re-used at different places in the
hierarchy.

We thought of this earlier, and found groupings pose their own set of challenges too.. Specifically:
- a groupings with leafrefs could not reference data nodes that reside in another grouping
- a grouping with leafrefs of relative path were challenge when the relative path references data nodes outside the grouping
- the augmentation of the grouping by other modules is not as straightforward

That said, the grouping proposal seems to

one could also think that with groupings one could address reuse of the a model (e.g. Ietf-interfaces) for logical devices or VM (see below). In fact, in your draft (section 2) you explicitly discourage this approach as not scalable solution


   With the "uses" approach, ietf-interfaces would have to define a
   grouping with all its nodes, and the new model for logical devices
   would have to use this grouping.  This is a not a scalable solution,
   since every time there is a new model defined, we would have to
   update our model for logical devices to use a grouping from the new
   model.  Another problem is that this approach cannot handle vendor-



We have a comment/concern/suggestion and we value your feedback.
The generic TE model currently references data nodes in the global
tree (e.g. from the ietf-interfaces model to define additional TE
properties associated with a specific device interface). Our
understanding after reading section 3.1 of your draft is the mounted
model can *not* reference any data nodes outside the scope of the
mount-point (e.g. global data nodes in the yang tree). This poses a
limitation for us, do you have a suggestion for this problem?
One possible solution we thought of was to replace the leaf-refs
pointing to the global data nodes (e.g. Ietf-interfaces) with context
names (e.g. the interface name).. This decouples the data-nodes
defined in the TE generic model from those in the global tree
(e.g. the actual interface ietf-interfaces model). Any feedback on
this or better suggestions?

If you use groupings instead, you can still use proper leafrefs.

Not in all cases — as described above.

Regards,
Tarek



/martin



Regards,
Tarek
Excerpt from draft-ietf-netmod-schema-mount
3.1<https://tools.ietf.org/html/draft-ietf-netmod-schema-mount-01#section-3.1>.
Augment and Validation in Mounted Data
    All paths (in leafrefs, instance-identifiers, XPath expressions, and
    target nodes of augments) in the data models mounted at a mount point
    are interpreted with the mount point as the root node, and the
    mounted data nodes as its children.  This means that data within a
    mounted subtree can never refer to data outside of this subtree.