Re: [Roll] knowing which multiple metrics matter: MRHOF related questions

Mukul Goyal <mukul@uwm.edu> Tue, 05 June 2012 06:33 UTC

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Date: Tue, 5 Jun 2012 01:33:32 -0500 (CDT)
From: Mukul Goyal <mukul@uwm.edu>
To: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
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Subject: Re: [Roll] knowing which multiple metrics matter: MRHOF related questions
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Hi Pascal

>An instance can use multiple metrics bound by some logic. But those metrics and that logic must be identical throughout the instance. A different (set of) metric means a different OF even if MRHOF is behind both.

Could you please explain. This is not how I understand the relationship among an RPL Instance, an OF and metrics to be. As per my understanding, there is one OF associated with an RPL Instance and the metrics in use are chosen by the root. Accordingly, an RPL Instance _can_ have different DODAGIDs using different sets of metrics.

Thanks
Mukul 




----- Original Message -----
From: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
To: "Michael Richardson" <mcr+ietf@sandelman.ca>
Cc: roll@ietf.org
Sent: Monday, May 28, 2012 2:04:59 PM
Subject: Re: [Roll] knowing which multiple metrics matter: MRHOF related questions

Hello Micheal

>>>>> "Pascal" == Pascal Thubert <(pthubert)" <pthubert@cisco.com>> writes:
    Pascal> I think RPL does not want to take party there. The OF is a
    Pascal> piece of logic to tie metrics and policies together.  

My question is:

- do the nodes of a DODAG have to use the same metrics to pick a parent,
  (and if so, how do they agree)
  I think that they do not, so long as they use an algorithm (such as
  MRHOF) which has certain properties.

[Pascal]  The current spec requires that all players of an instance play by the same rule, that is same OF, in order to guarantee the expected result. 
The goal there was deeper than the OF, like same OF with same parameterization. MRHOF is generic and allows various incarnations, depending in the metrics; all those incarnations are to be seen as different OFs WRT to RPL 's uniqueness rule.

Note that this rule is not for looplessness, the Rank would assure that anyway.


- if we had multiple RPL instances in an LLN, using different metrics,
  then we would have multiple RPL Instances and DODAGs.  The different
  set of metrics would not co-exist in the same RPL Instance.

[Pascal] I think we agree. An instance can use multiple metrics bound by some logic. But those metrics and that logic must be identical throughout the instance. 
A different (set of) metric means a different OF even if MRHOF is behind both.

Cheers,

Pascal

-- 
Michael Richardson <mcr+IETF@sandelman.ca>ca>, Sandelman Software Works 
IETF ROLL WG co-chair.    http://datatracker.ietf.org/wg/roll/charter/

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