Re: [Roll] enrollment priority

Michael Richardson <mcr+ietf@sandelman.ca> Fri, 10 April 2020 21:44 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
To: Routing Over Low power and Lossy networks <roll@ietf.org>
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Date: Fri, 10 Apr 2020 17:44:11 -0400
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Subject: Re: [Roll] enrollment priority
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Rahul Jadhav <rahul.ietf@gmail.com> wrote:
    > The draft should make it clear that the min-priority can only be
    > reduced or kept the same (i.e., the min-priority absolute value can
    > only increase, thus reducing priority) going downstream in the
    > sub-DODAG. For e.g., if a 6LR sets the min-priority to 10, the
    > downstream 6LRs can only set the value >= 10 in their min-priority.

I will add, to section 2:

    > Consider the following scenario (the attached image will help explain):
    > Node C's preferred parent is node B and alt parent is node A. What
    > happens if the min-priority of node B changes to 127 after the node C
    > has joined the network with B as a preferred parent? I am assuming
    > that the min-priority impacts only the newly joining nodes i.e., once
    > node B changes the priority to 127 then node C also changes the
    > priority to 127 and thus no new nodes join. However, the change in
    > node B's min-priority may not impact node C's decision to continue
    > using node B as the preferred parent (?). What happens when node C has
    > alternate parent node A whose priority is lesser than 127. Should node
    > C switch to node A so as to allow new downstream nodes to join the
    > network?

First, I think that if node C had a MP of 127, then it would *NOT* advertise
itself as a Join Proxy.  So if it was sending Enhanced Beacons, then it
would:
  a) set proxy prio to 0x7f.
  b) maybe even stop sending EBs [have to think about this]
  c) not accept any unencrypted traffic!

so yeah, node D would be out of luck.

If node C *does* change to node A, then it would change it's priority.
But it has to do that, and maybe there are reasons why A won't accept it.

    > I don't believe there would be backward compatibility issues with this
    > new option, but the draft can clarify this explicitly.

I have added:

  This document uses the extensions mechanism designed into {{!RFC6550}}.
  It does not need any mechanism to enable it.

  6LRs that support this option, but whose parent does not send it SHOULD
  assume a value of 0x40 as their base value.
  The nodes adjust this base value based upon their observed congestion,
  emitting their adjusted DIO value to their children.


(0x40 is my wet-finger-in-the-air, seat-of-the-pants guess)

    > We tried working on a problem-statement in the past in LWIG [1] which
    > talked about how to handle the case where some upstream nodes cache
    > (routing/neighbor) is full and how would it stop the new downstream
    > nodes to attach in that path. Enrollment priority was referenced as a
    > possible solution.

Good.  Would more informative cross-references help?
Do you want to co-author?

--
]               Never tell me the odds!                 | ipv6 mesh networks [
]   Michael Richardson, Sandelman Software Works        |    IoT architect   [
]     mcr@sandelman.ca  http://www.sandelman.ca/        |   ruby on rails    [


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Michael Richardson <mcr+IETF@sandelman.ca>ca>, Sandelman Software Works
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