Re: [babel] Martin Duke's No Objection on draft-ietf-babel-yang-model-10: (with COMMENT)

[Mahesh Jethanandani <[e-mail]>]

Hi Martin,

You bring up a point that I have been struggling with.

> On May 17, 2021, at 2:10 PM, Martin Duke via Datatracker <[e-mail]> wrote:
> 
> Martin Duke has entered the following ballot position for
> draft-ietf-babel-yang-model-10: No Objection
> 
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> 
> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about DISCUSS and COMMENT positions.
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> 
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-babel-yang-model/
> 
> 
> 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> (2.3)
> 
> leaf received-metric {
>           type uint16;
>           description
>             "The metric with which this route was advertised by the
>              neighbor, or maximum value (infinity) to indicate the
>              route was recently retracted and is temporarily
>              unreachable. This metric will be 0 (zero) if the route
>              was not received from a neighbor but was generated
>              through other means. At least one of
>              calculated-metric or received-metric MUST be non-NULL.";
>           reference
>             "RFC ZZZZ: Babel Information Model, Section 3.6,
>              RFC 8966: The Babel Routing Protocol, Section 2.1.";
>         }
> 
>         leaf calculated-metric {
>           type uint16;
>           description
>             "A calculated metric for this route. How the metric is
>              calculated is implementation-specific. Maximum value
>              (infinity) indicates the route was recently retracted
>              and is temporarily unreachable. At least one of
>              calculated-metric or received-metric MUST be non-NULL.";
>           reference
>             "RFC ZZZZ: Babel Information Model, Section 3.6,
>              RFC 8966: The Babel Routing Protocol, Section 2.1.";
>         }
> 
> I don't understand the relationship between these two. If the metric was
> calculated rather than received, why would the value be zero instead of NULL?
> Isn't a zero metric dangerous in a routing algorithm?

How do you represent NULL? By definition, NULL means there is no value, that we are looking at an empty leaf. A ‘received-metric’ per my reading, cannot be empty. It is either received from a neighbor, or is a generated value, and therefore cannot be empty. Same is the case for ‘calculated-metric’. Combine this with the fact that the info model defines the leaf of type unsigned int 16, and thus the value of 0. Would it help to add something to this effect?

"A value of 0 implies a NULL value, and SHOULD NOT be used in metric calculation".

Alternatively, one could change the type to signed int 16, something the TR-181 model does. In that case the value has two meanings. If the value is less than 0, it means the value is NULL, but if the value is greater than 0, then it carries the actual metric value??

In either case, an implementor cannot take the value of the metric as is and use it, without checking if it is 0 or -1.

> 
> (4) "config true perspective"

Dropped the phrase from the sentence.

Thanks

Mahesh Jethanandani
[e-mail]