Re: [netmod] Question on intefaces-state model

On 14/06/2017 09:28, Ladislav Lhotka wrote:
>> On 14 Jun 2017, at 00:35, Alex Campbell <Alex.Campbell@Aviatnet.com> wrote:
>>
>>
>> Presumably a device is free to not implement an optional config=false node if that node would never be returned in a response anyway - as this will make no externally visible difference.
> That's my view, too. However, this reasoning works if the parent container is being retrieved but it is not very clear what is supposed to happen if the client asks explicly for that optional parameter. This looks like a gap in the architecture – most of the time, YANG pretends to be something like a schema language in that it describes constraints on a valid data tree but conformance issues like what parameters a server needs to implement are something different.
It would surely do the same thing as if a client requested any node in a 
YANG data tree that doesn't exist.

RESTCONF has special semantics if a GET request is for a specific leaf 
rather than a parent container, but I just regard this as a mistake in 
the specification (i.e. the "restconf default handling" thread on the 
NETCONF alias).

>
>> However, if the model states or implies that the node is present under certain conditions (for example, the node is always present for Ethernet ports), and the device can meet those conditions (i.e. it has an Ethernet port), then the device must implement the node or it does not conform to the model.
> Right, this could be written in a description, but I've been assuming it is not the case.
>
> Lada
Rob

>
>>
>>
>> Alex
>>
>> From: netmod <netmod-bounces@ietf.org> on behalf of Andy Bierman <andy@yumaworks.com>
>> Sent: Wednesday, 14 June 2017 7:30 a.m.
>> To: Ladislav Lhotka
>> Cc: netmod@ietf.org
>> Subject: Re: [netmod] Question on intefaces-state model
>>
>>
>> On Tue, Jun 13, 2017 at 11:52 AM, Ladislav Lhotka <lhotka@nic.cz> wrote:
>> Andy Bierman <andy@yumaworks.com> writes:
>>
>>> On Mon, Jun 12, 2017 at 10:21 AM, Juergen Schoenwaelder <
>>> j.schoenwaelder@jacobs-university.de> wrote:
>>>
>>>> On Fri, Jun 09, 2017 at 10:55:20AM +0000, Bogaert, Bart (Nokia -
>>>> BE/Antwerp) wrote:
>>>>> We have a question regarding the statistics container as defined in the
>>>>> interfaces-state model.  This container defines one mandatory leaf
>>>>> (discontinuity-time) while all other leafs are optional.  What is the
>>>>> rationale behind this leaf being mandatory and not an optional field?
>>>>>
>>>>> It does not make a lot of sense to return a discontinuity-time value and
>>>> no
>>>>> counters if none of the counters are relevant for a specific interface.
>>>>>
>>>>> Another alternative could be to add, via a deviation, a when clause to
>>>> the
>>>>> container indicating for which ifType(s) the container is (or is not)
>>>>> present. But that would lead to not supporting the IETF interfaces model
>>>> to
>>>>> the full extent.
>>>>>
>>>> The discontinuity-time is relevant for _any_ counter associated with
>>>> an interface, regardless whether the counter is defined in the
>>>> interfaces model or elsewhere. I have a hard time to imagine an
>>>> interface that has zero counters.
>>>>
>>>>
>>> The mandatory-stmt is very confusing for config=false nodes. Mandatory=true
>>> means
>>> an <rpc-reply> must contain an instance of the mandatory leaf.
>> I don't think it is that confusing. RFC 7950 defines what a valid data
>> tree means and "mandatory" are among the constraints.
>>
>> I agree though that in terms of a management protocol it means different
>> things for config true and false data, but this is true also for default
>> values and maybe other YANG concepts as well.
>>
>>> Mandatory=false does not mean optional-to-implement although it sure
>>> looks that way for config=false nodes.  Only if-feature can make a node
>>> optional to implement.
>> I don't think this interpretation is supported by any text in the YANG
>> spec. State data nodes that are optional needn't be implemented.
>>
>>
>> RFC 7950, sec 5.6  (Conformance) does not support your interpretation.
>> It defines basic behavior, optional (via features), and deviations as the only mechanisms affecting conformance.
>>
>>
>> Lada
>>
>>
>> Andy
>>   
>>>
>>>   /js
>>>
>>>
>>> Andy
>>>
>>> --
>>>> Juergen Schoenwaelder           Jacobs University Bremen gGmbH
>>>> Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
>>>> Fax:   +49 421 200 3103         <http://www.jacobs-university.de/>
>>>>
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>> --
>> Ladislav Lhotka, CZ.NIC Labs
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>>
>>
> --
> Ladislav Lhotka, CZ.NIC Labs
> PGP Key ID: 0xB8F92B08A9F76C67
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