Re: [netmod] Last Call: <draft-ietf-netmod-entity-07.txt> (A YANG Data Model for Hardware Management) to Proposed Standard

On Thu, Jan 04, 2018 at 12:37:02PM -0000, tom p. wrote:
> This I-D illlustrates a problem that the netmod WG has been wrestling
> with for a decade or more and (IMHO) has yet fully to solve.  This I-D
> is in Last Call at the same time as netmod-revised-datastores, which
> proposes the most recent solution.
> 
> The problem is that an 'object' can take more than one value and the
> question is how to represent that.  Previously it was by having twin
> trees in the schema; now it is by having one common schema and multiple
> datastores, <running> for the user-supplied values, <operational> for
> those actually in use, the latter including values learnt from the
> hardware or protocols or some other means.
> 
> In this model the situation arises with 'serial-num' which may take a
> value from the hardware or may be written as part of configuration (the
> idea of configuring serial numbers may seem unusual but has been
> inherited from the Entity MIB [RFC6933] with the endorsement of the
> netmod WG).
> 
> So if there is an accessible hardware value and the user writes a value,
> who wins?  There is no way of specifying this, neither in YANG nor in
> 'revised-datastores'; in this case, the 'description' specifies that the
> value determined from the component should be used so a user can
> configure a value, see it in the <running> datastore but not in the
> <operational> datastore and cannot tell why, unless they are familiar
> with the minutiae of description clauses.

The origin attribute will at least tell you where the operationally
used value is originating from.

> In this case, the model is clear as to which value, configured or
> learnt, wins; here, it is learnt.  In other cases, it may be that
> configured wins or that may be something a user wants to influence as
> part of policy.
> 
> Is that still one schema or is it now two slightly different schemas
> based on the description clause coupled with the datastore that the
> schema is being applied to?
> 
> Is the description clause an adequate way of expressing policy?
> 
> In passing, routing protocols addressed this dilemma a long time ago,
> with concepts such as 'Admin Distance'.
> 
> (Whether or not this particular value should be configurable is a
> different and irrelevant discussion; the MIB WG decided it should be,
> the YANG WG likewise - and there are plenty of other objects which
> illustrate this dilemma, how to specify who wins).
> 
> Tom Petch
> 
> ----- Original Message -----
> From: "The IESG" <iesg-secretary@ietf.org>
> Sent: Thursday, December 21, 2017 10:22 PM
> 
> > The IESG has received a request from the Network Modeling WG (netmod)
> to
> > consider the following document: - 'A YANG Data Model for Hardware
> Management'
> >   <draft-ietf-netmod-entity-07.txt> as Proposed Standard
> >
> > The IESG plans to make a decision in the next few weeks, and solicits
> final
> > comments on this action. Please send substantive comments to the
> > ietf@ietf.org mailing lists by 2018-01-10. Exceptionally, comments may
> be
> > sent to iesg@ietf.org instead. In either case, please retain the
> beginning of
> > the Subject line to allow automated sorting.
> >
> > Abstract
> >
> >    This document defines a YANG data model for the management of
> >    hardware on a single server.
> >
> > The file can be obtained via
> > https://datatracker.ietf.org/doc/draft-ietf-netmod-entity/
> >
> > IESG discussion can be tracked via
> > https://datatracker.ietf.org/doc/draft-ietf-netmod-entity/ballot/
> >
> > No IPR declarations have been submitted directly on this I-D.
> 
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-- 
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/>