[Roll] Ralph Droms' Discuss 1 on draft-ietf-roll-of0-15
"Pascal Thubert (pthubert)" <pthubert@cisco.com> Tue, 09 August 2011 16:59 UTC
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Date: Tue, 09 Aug 2011 18:59:23 +0200
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Thread-Topic: Ralph Droms' Discuss 1 on draft-ietf-roll-of0-15
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From: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
To: Ralph Droms <rdroms.ietf@gmail.com>, The IESG <iesg@ietf.org>
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Cc: roll@ietf.org, draft-ietf-roll-of0@tools.ietf.org, roll-chairs@tools.ietf.org
Subject: [Roll] Ralph Droms' Discuss 1 on draft-ietf-roll-of0-15
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> 1. I would like to discuss the interoperability and deployability of > nodes in a RPL Instance that uses OF0. If I understand section 4.1 > correctly, a node is free to compute step-of-rank using any method it > chooses. There is a suggestion that using link properties for the > computation is preferred, but the specific link properties and the > method of computation are not specified. Later, section 4.1 defines > two parameters, rank_factor and stretch_factor, that modify > step_of_rank to generate a "stretched step_of_rank". The latter value > is then multiplied by the MinHopRankIncrease to compute the node's > rank_increase. > > My concern is the degrees of freedom provided to the implementation in > the computation of rank_increase. I'm hoping I can get an explanation > of some reason to believe that independent implementations, using > different methods to compute step_of_rank and potentially different > configured values for rank_factor and stretch_factor, will > interoperate successfully to form an operational network. > [Pascal] Success is defined as reaching the Goal (a set of nodes or the Internet), using the best links possible. OF0 does not guarantee a particular optimization for a certain metric, but trusts the implementation to rank a usable link from 1 to 9. Section 6.2 indicates that , rank_factor and stretch_of_rank are configured parameters. Maybe section 7.1 should indicate that a deployment should ensure they are homogeneous? > More specifically, if a simple metric that leads to a hop-count > computation for path lengths and route computation is known to yield > unusable performance, why is it not required that a node use some sort > of computation based on link characteristics? And, if the computation [Pascal] this is RECOMMENDED for links such as radio in section 4.1. But RPL (and OF0) also apply to other links, including 802.3 and 802.11. If you have a switched fabric as backbone, hop count or similar is appropriate there. > of the step_factor is entirely up to the node, why are the rank_factor > and stretch_factor needed? Why not just leave the entire computation > to the implementation, with limitations that the resulting step_factor > lie in the range MINIMUM_STEP_OF_RANK and > MAXIMUM_STEP_OF_RANK? [Pascal] There is only one factor, the rank_factor. It is used to discriminate different link types. Eg in homenet, 802.15.4 could have a huge rank_factor (3 or 4), a WIFI would use say 2 and G/ Ethernet 1. This is a parameter that must be configured the same for all nodes for a given type of link and it has a consequent action on the rank_increase regardless of implementation. The result is that the topology OF0 builds can be deployed so as to limit the wireless hops, and favor .11 over .15.4. I think that is what you are really asking for and thus we are probably on the same line. What do you think? Pascal
- [Roll] Ralph Droms' Discuss 1 on draft-ietf-roll-… Pascal Thubert (pthubert)