Then, what would be the difference between an optional and a mandatory data 
node?

In some RFC8795 implementations, the support or not support of a data node may 
also depend on the specific network-type so in this case the data node cannot 
be marked as not-supported

Italo

From: Andy Bierman [mailto:[email protected]]
Sent: mercoledì 19 maggio 2021 19:35
To: Italo Busi <[email protected]>
Cc: [email protected]; [email protected]
Subject: Re: [netmod] Question about optional data nodes in YANG



On Wed, May 19, 2021 at 10:12 AM Italo Busi 
<[email protected]<mailto:[email protected]>> wrote:
We have got a question about how to deal with YANG optional data nodes and in 
particular how can a client know which optional data node has been implemented 
by a server.

We think that there is no issue with config=false data nodes. When the client 
retrieves a YANG tree from the operational datastore it will not get the data 
nodes that are not implemented by the server, as reported in section 5.3 of 
RFC8342:

If no value is returned for a given node, then
this implies that the node is not used by the device.

Is our understanding correct?



The server is expected to advertise a deviation for not-supported for every 
subtree
it does not implement.

https://datatracker.ietf.org/doc/html/rfc7950#section-7.20.3.2




       deviation /base:system/base:daytime {

         deviate not-supported;

       }


Andy



The doubt we have is about the config=true data nodes.

How can the client know whether the server supports the configuration of an 
optional config=true data node before trying to configure them and getting an 
error message?

We understand that it is possible to know whether a YANG model or a feature of 
the YANG model (i.e. a group of data nodes) is supported by the server. The 
question is rather on specific data nodes with config=true.

We have found scenarios where it could be useful to implement a sub-set of 
optional data nodes (profile) of an IETF standard YANG model, but it is not 
very clear how a client can understand which profile has been implemented by 
the server.

Some examples of profiles of an IETF standard YANG model are provided in:

https://datatracker.ietf.org/doc/html/draft-busi-teas-te-topology-profiles

Thanks, Aihua and Italo

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