Re: [OPSAWG] [netmod] Question on how to design a Yang model to reflect auto-asignment of a give leaf

[Oscar González de Dios <[e-mail]>]

-----Mensaje original-----
De: Martin Bjorklund <[e-mail]>
Enviado el: martes, 11 de febrero de 2020 11:00
Para: Oscar González de Dios <[e-mail]>
CC: [e-mail]; [e-mail]
Asunto: Re: [netmod] Question on how to design a Yang model to reflect auto-asignment of a give leaf

Hi,

Oscar González de Dios <[e-mail]> wrote:
> Dear OPSAWG and Netmod colleagues,
>
>                 During last IETF Opsawg meeting we raised a question (and
>                 there was some discussion during the meeting) that we have
>                 found yet no good answer and we would like to discuss it with
>                 operations and Yang experts.
>
>                 The use case is the following: We have a yang module which
>                 holds certain optional leafs. The behaviors that we would like
>                 to have (and distinguish between them) are:
>
>
> a) The user does not provide the value and such value is auto-assigned
> by the system (a device (if it is a device module) or a controller (if
> it is a network/service module)).

I assume that this value not a static default value?

[Oscar] True. Should the leaf have a default value, it implies that "if the value is not set, the default value is taken".

> b) The user does not provide a value and wants that such value IS NOT
> set by the system (as assigning a value has implications). That is,
> intentionally it is aimed at being left "empty" and should not be
> expanded. So, either the value is set or should remain empty

Do you mean that you want (a) and (b) at the same time for the same leaf?

[Oscar] No. Depending on the leaf, we would like to specify behavior a or behavior b. Behavior a is ok for most of the cases.  The problem is that in some cases, assigning a value has way more implications and the service will not work properly. Those case are the ones we wanted to specifically tackle.

> What is the best way to model this behavior? I see that some yang
> modules have added an "auto-assignment" leaf to express if
> auto-assignment is desired or not. (hence, auto-assignment false, and
> leaf not set, would do not assign).
>
> Which is the "default" rule for a leaf that is not set? It is that the
> system is free to create it (via template or any means of
> auto-assignment) or should leave it as is, that is, empty?
>
> In NMDA, the system is allowed to expand a given configuration. This
> fact, in my personal view, implies that by "default" any system could
> implement the "auto-assignment" behavior being compliant with
> Neconf/Restconf/NMDA rules (but I am not sure if the interpretation is
> correct).

There are (at least) three ways to interpret "auto-assign".  The client writes to running, and then the server auto-assigns X:

  (a) in running
  (b) in intended
  (c) in the operational state

(c) is uncontroversial and simple to implement in all servers, and simple to understand.
[Oscar] agree

(b) is allowed by NMDA but requires more of the server implementation; specifically it requires the server to support that intended is different from running.
[Oscar] Agree . "Theoretically speaking" this is the behavior I would consider strictly follows NMDA guidelines. Reality is implementations are yet far from this...

(a) is not recommended in general; running should be fully owned by the client(s) and not modified by the server.
[Oscar] Agree.

[Oscar] So... what would be the best way to specify the behavior? Explicitly adding an auto-assign leaf to identify the behavior? Just "obey" NMDA rules?


/martin



>
> Best Regards,
>
>                 Óscar
>
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