Re: [Roll] Ralph's DISCUSS on MRHOF spec

Philip Levis <pal@cs.stanford.edu> Fri, 08 June 2012 22:35 UTC

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Subject: Re: [Roll] Ralph's DISCUSS on MRHOF spec
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On Jun 7, 2012, at 2:49 PM, Pascal Thubert (pthubert) wrote:

> 
> [Pascal] I agree with Ralph here. I fail to be convinced that there will an explosion of OCPs if we fail to factorize the metrics. But I see how the device implementation can be simplified if the OCP says it all. Also, we do not want to force a device that implements MRHOF to have to implement all metrics in the I-D. Conversely, say we extend MrHof to other metrics with further work, wouldn't it become OCP 2 anyway? I wouldn't mind blocking OCP 1..9 for current and future MrHof metric variations.


Well, here was your argument for it:

Begin forwarded message:

> From: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
> Subject: RE: rank and metric in DIOs
> Date: November 6, 2009 12:52:54 AM PST
> To: "Jonathan Hui" <jhui@archrock.com>om>, "Richard Kelsey" <richard.kelsey@ember.com>
> Cc: <rpl-authors@external.cisco.com>
> 
> Hi Jonathan:
> 
> I do not think we need to enforce anything. The gear box protects the
> future. I think we must leave the core spec abstract on that. And since
> the burden is on the OCP anyway, what's the point?
> 
> OTOH, we can design an OCP / metric where the metric has the properties
> that is proposed here. In that case, the OCP should be simpler. OCP zero
> is a bit like that if you look at it :) Maybe our disagreement is only
> what is in the base spec (the generic thing) and what goes in a specific
> instantiation (the OCP).
> 
> I can wholeheartedly accept Richard's way of start simple and solid, and
> then build over it if that's how more people can catch up with our work
> for both understanding and implementation. As a next step for an OCP>0,
> it is certainly a good thing to do. I'd support that, even more gladly
> if we base it on ETX. 
> 
> Personally I think that this proposal yields limitations that will soon
> appear inacceptable for a number of use cases where you have more
> metrics, not necessarily incremental, fast varying, and logic to wrap
> that up. So we MUST NOT make it generic. Some of those metrics you need
> to propagate, but you do not want to change the rank and restart
> trickle.
> 
> I listed the properties of the rank several times already. Its stability
> (long time constant is only one of those properties). 
> 
> A list (probably incomplete) of the differences:
> 
> - its type: it is a scalar
> - its function: it is a relative coordinate, not a cost or a distance to
> root.
> - its granularity: it is coarse grained to enable siblings
> - its range with a short infinite
> - its properties: rank is strictly monotonic, and increments from parent
> rank + OCP stuff, not like the metrics themselves increment or
> propagate. How would you play with throughput or jitter?
> - universal and very abstract: we can always default to OCP0. I have
> uses cases where I could actually do that successfully, like cars in a
> parking lot.
> 
> I think that the OCP job to turn its knowledge of the requirements, the
> environment and the ranks of parents into a rank cannot be generalized.
> Leaving that operation out of RPL base spec is actually making it
> simpler.
> 
> I'd be glad to add some words in RPL base spec on those rank properties
> though...


Phil