[Roll] Ralph Droms' Discuss on draft-ietf-roll-of0-15: (with DISCUSS and COMMENT)

[Ralph Droms <[e-mail]>]

Ralph Droms has entered the following ballot position for
draft-ietf-roll-of0-15: Discuss

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DISCUSS:
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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.

More specifically, if a simple metric that leads to a hop-count
computation for path lengths and route computation is know 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
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?

2. I would also like to discuss the requirement for a
mandatory-to-implement OF, whether OF0 is that mandatory-to-implement
OF and where the requirement to implement OF0 is specified.

3. In section 7.1, why is support of the DODAG Configuration option
only a SHOULD?

   An OF0 implementation SHOULD support the DODAG Configuration option
   as specified in section 6.7.6 of [I-D.ietf-roll-rpl] and apply the
   parameters contained therein.

4. This point is more editorial than completely technical, but I list
it as a Discuss because of the potential for confusion among
implementors.  I found the use of "DODAG Version" a little confusing,
because it appears sometimes to mean one of the successive Versions of
one DODAG and at other times to mean a specific DODAG chosen from
several DODAGs.  For example, from the Introduction:

   RPL forms Directed Acyclic Graphs (DAGs) as collections of
   Destination Oriented DAGs (DODAGs) within instances of the protocol.
   Each instance is associated with a specialized Objective Function.  A
   DODAG is periodically reconstructed as a new DODAG Version to enable
   a global reoptimization of the graph.

   An instance of RPL running on a device uses an Objective Function to
   help it determine which DODAG Version it should join.  The OF is also
   used by the RPL instance to select a number of routers within the
   DODAG Version to serve as parents or as feasible successors.

In my opinion, the first use of "DODAG Version" refers to a sucessive
Version of one DODAG, while the second use refers to a selection of
one DODAG from many DODAGs.  Would it be possible to just use "DODAG"
for the second intended use?  Or am I confused about the intended
meanings?

5. In section 4.2.1, what does it mean to "validate a router"?  Why
would a router that passes validation ("succeeded that validation
process") only be "preferable"?



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COMMENT:
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1. I consider the following comment to be a technical rather than an
editorial suggestion because of the redundancy and potential conflict
between the text in the Introduction of this document and the contents
of draft-ietf-roll-rpl-19.  It would improve the document to edit the
Introduction down to a paragrpah that combines the following sentence
from the first paragraph with the content from the last paragraph:

   An Objective Function
   defines how a RPL node selects and optimizes routes within a RPL
   Instance based on the information objects available.

Editing out the text describing RPL would also presumably address
Stewart's Discuss about the use of multiple OFs in one RPL Instance.

2. I don't understand this phrase from the last paragraph of the
Introduction:

   [...] OF0 enforces normalized values for the rank_increase of a
   normal link and its acceptable range

Do I have it right that a node uses OF0 to determine the rank_increase
for a successor within the range:

   MINIMUM_STEP_OF_RANK < stretched step_of_rank < MAXIMUM_STEP_OF_RANK

Unless I'm missing something, these limits on the stretched
step_of_rank enforce (indirectly) a range on rank_increase, but don't
normalize rank_increase by some normalizing factor.

3. Is there a reason to switch between "successor" and "parent"
throughout the document?  For example, the title of section 4.2 is
"Feasible Successors Selection" and the title of the immediately
following section 4.2.1 is "Selection Of The Preferred Parent".  Are
"successor" and "parent" are synonymous in this context?  Would it be
asking for foolish consistency to choose one or the other?

4. In section 7.1, what is "build-time"?

   At build-time [...]

Also in section 7.1, what is a "fixed constant" (as opposed to any
other kind of "constant").  Why are overridden values - I assume these
are overriden by some administrative method like CLI configuration or
some management protocol? - only applied at the next Version of the
DODAG?

5. In section 3, how is "good enough" defined?  From reading the rest
of the spec, I don't see where any assessment of quality is applied to
ensure that "good enough" is more than basic connectivity.  I suggest
just dropping "good enough".