Re: [babel] [netmod] NULL value for uint16

[Mahesh Jethanandani <[e-mail]>]

[Bringing in babel WG, instead of maintaining two threads]


Hi Juergen,


> On Sep 10, 2021, at 1:09 PM, Jürgen Schönwälder <[e-mail]> wrote:
> 
> On Fri, Sep 10, 2021 at 01:40:03PM -0400, Lou Berger wrote:
> 
>> and it references an RFC that says:
>> 
>>      This is a 16-bit unsigned integer;
>>      if the data model uses zero (0) to represent NULL values for
>>      unsigned integers, the data model MAY use a different data type
>>      that allows differentiation between zero (0) and NULL.
> 
> We are talking about RFC 9046? RFC 9046 repeats this sentence several
> times but I find it difficult to infer the intended meaning. If 0 is a
> legal value, you can't have it represent "NULL" at the same time.

There are five definitions in the Babel YANG model <https://www.ietf.org/archive/id/draft-ietf-babel-yang-model-11.html> that use NULL to represent a special meaning.

With ‘babel-route-received-metric’, and ‘babel-route-calculated-metric’ values use NULL or 0 to represent that a route was NOT received. A non-zero value indicates that a route was received and subsequently advertised.

With ‘babel-expo-mcast-hello-seqno’, and ‘babel-exp-ucast-hello-seqno’ a value of NULL or 0 is used to represent that a multicast, or unicast hello packets respectively are NOT expected or processing of multicast/unicast packets is not enabled. Although not explicit stated, it also means that the sequence number cannot be a 0.

Finally, with ‘babel-ucast-hello-seqno’ a value of NULL or 0 is used to represent a unicast hello is not being sent. A non-zero value reflects the current sequence number in use for unicast hellos. Again, although not explicit, it also means that the sequence number cannot be a 0.

> 
> In YANG, we tend to not instantiate leafs that do not have a value.
> Anyway, if a YANG author wants to report a special value to indicate
> that there is no value, then you have design for it and reserve and
> set aside a special value from the number space or use a union.

This YANG model reserves the value of 0 to indicate that these leafs are not set or the particular attribute is not enabled.

Thanks.

> 
> RFC 9046 uses the same text for things like sequence numbers. Skimming
> RFC 8966, it seems sequence numbers are 16-bit integers:
> 
>   Sequence numbers (seqnos) appear in a number of Babel data
>   structures, and they are interpreted as integers modulo 2^(16).
> 
> The text in the context makes me believe they are an unsigned 16-bit
> integers. If so, there simply is no way to use 0 to indicate that a
> sequence number is absent. The problem really might be the text in RFC
> 9046.
> 
> /js
> 
> -- 
> Juergen Schoenwaelder           Jacobs University Bremen gGmbH
> Phone: [phone]         Campus Ring 1 | 28759 Bremen | Germany
> Fax:   [phone]         <https://www.jacobs-university.de/>
> 
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Mahesh Jethanandani
[e-mail]