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

"Tarek Saad (tsaad)" <> Mon, 09 May 2016 15:44 UTC

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From: "Tarek Saad (tsaad)" <>
To: "Robert Wilton -X (rwilton - ENSOFT LIMITED at Cisco)" <>
Thread-Topic: [netmod] Use of schema mounts for common model
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Date: Mon, 09 May 2016 15:44:22 +0000
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Cc: "" <>, Martin Bjorklund <>, "" <>, "" <>, "" <>, "" <>
Subject: Re: [mpls] [netmod] Use of schema mounts for common model
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Hi Robert,

Thanks. Please see inline…

From: "Robert Wilton -X (rwilton - ENSOFT LIMITED at Cisco)" <<>>
Date: Tuesday, May 3, 2016 at 10:34 AM
To: Tarek Saad <<>>
Cc: Martin Bjorklund <<>>, "<>" <<>>, "<>" <<>>, "<>" <<>>, "<>" <<>>, "<>" <<>>
Subject: Re: [netmod] Use of schema mounts for common model

Hi Tarek,

I'm also wondering whether using schema mount in this way might be a bit like using a sledgehammer to crack a nut.

With interfaces, there is a per device global list of interfaces where each list entry can have a different type and different properties.  Currently these are using the iana:iftype to differentiate their different schema modes, but there is an idea to use more abstract identities.

I'm not really familiar with the tunnel YANG models, but I was wondering whether the same approach has been considered for the TE tunnel YANG models at all?

I.e. rather than having a separate protocol instantiation for each different data plane technology, instead have a per device global list of tunnels that hold the configuration and operational state, making use of when statements and identities (if required) to manage dataplane specific leaves.  Each protocol specific data plane module could also maintain a protocol specific list containing leaf-refs back to the appropriate tunnels in the master list.

[TS]: Currently the TE tunnel model has a global list of interfaces. The team has discussed the idea of continuing to have a single global list and then assigning the tunnel technology type on per tunnel. This implicates the following:

  1.  There will be 1 full list of TE tunnels for all technology types. The list will be keyed by {(tunnel name)+(tunnel-technology-type)}. On nodes that may support multiple technology types (e.g. Packet and optical), the tunnels/LSPs for multiple technologies will be intermixed in the same list.
  2.  The same goes for the list of LSPs (instantiations of tunnels on ingress/transit/egress nodes.. In this case, such LSPs of different technology parts will be intermixed too.

This is possible, but may not be most elegant way of presenting this.. We were hoping it will be possible to have the segregation on per technology by mounting the tunnel list under the respective technology (packet, otn, etc.)..
Lastly, playing devil’s advocate :), one could argue it’s also possible to have one global list of interfaces in the interfaces model and make the device/VM an attribute (and part of the key) of the interface list.. That is an interface name foo can appear multiple times in the interface list (once per VM or physical).. Of course, IMO the mounting proposal is more elegant in this case..



On 29/04/2016 17:17, Tarek Saad (tsaad) wrote:
Thanks Martin, please see inline..

On 2016-04-29, 6:29 AM, "Martin Bjorklund" <<><>> wrote:

"Tarek Saad (tsaad)" <<>> 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,
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.:

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

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.



Excerpt from draft-ietf-netmod-schema-mount
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.

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