Subject: [Roll] Ralph Droms' Discuss 1 on draft-ietf-roll-of0-15

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Tue, 9 Aug 2011 18:59:23 +0200

> 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