Subject: [Roll] Question on path cost propagation in the DODAG

From: <[email-masked]>
Date: Mon, 2 Aug 2010 22:31:18 +0200

Hello Phil, all,

Our mutual questions and ensuing discussion at the IETF78 meeting in
Maastricht uncovered a different understanding of the way path costs are
propagated in the DODAG.
To the whole Working Group benefit, here's the discussion again.

Let's suppose that node X has selected node A as its preferred parent,
and nodes B and C as alternate parents. In addition, let's suppose node
Y is a child for node X.
During the life of the network, path costs propagated by DIOs are likely
to fluctuate while the ranks and parenthood relationship are kept more
stable, by only updating them on persistent and significant changes in
costs (see [1]).
The question is, what metric should X advertise in its own DIOs to Y?
* answer 1)
X should always advertize the cost derived from that of node A, no
matter what B and C advertize, because A is the preferred parent of X.
* answer 2)
X should advertize the best cost derived from its complete current
parent set, as in a distance vector routing protocol. Because B and C
are likely to be used as next-hops in addition to A (low-power and lossy
links come and go), it is right to somehow take them into account in the
cost propagated to Y.
* answer 3)
This decision is left to the implementer.

I couldn't find in RPL -11 any text providing an answer to this
question.
I'm curious to hear opinions from the Working Group, notably those who
have implemented RPL?
Note that in RPL-11, _the_ preferred parent can actually be a set of
parents [2].
Thanks,

Dominique

-----------------------
References

[1] RPL-11 Section 3.6
"The stability of the rank determines the stability of the routing
topology. Some dampening or filtering is RECOMMENDED
to keep the topology stable, and thus the rank does not necessarily
change as fast as some link or node metrics
would. A new DODAG Version would be a good opportunity to reconcile the
discrepancies that might form over time between
metrics and ranks within a DODAG Version."

[2] RPL-11 Section 8.2.1.
"The preferred parent is conceptually a single parent although it may be
a set of multiple parents if those parents are
equally preferred and have identical rank."

---

From: Philip Levis <[email-masked]>
Date: Mon, 2 Aug 2010 13:43:57 -0700

On Aug 2, 2010, at 1:31 PM, <[email-masked]> <[email-masked]> wrote:

> Hello Phil, all,
> 
> Our mutual questions and ensuing discussion at the IETF78 meeting in Maastricht uncovered a different understanding of the way path costs are propagated in the DODAG.
> 
> Let's suppose that node X has selected node A as its preferred parent, and nodes B and C as alternate parents. In addition, let's suppose node Y is a child for node X.
> 
> During the life of the network, path costs propagated by DIOs are likely to fluctuate while the ranks and parenthood relationship are kept more stable, by only updating them on persistent and significant changes in costs (see [1]).
> 
> X should always advertize the cost derived from that of node A, no matter what B and C advertize, because A is the preferred parent of X.
> 
> * answer 2) 
> X should advertize the best cost derived from its complete current parent set, as in a distance vector routing protocol. Because B and C are likely to be used as next-hops in addition to A (low-power and lossy links come and go), it is right to somehow take them into account in the cost propagated to Y.
> 
> * answer 3) 
> I couldn't find in RPL -11 any text providing an answer to this question. 
> I'm curious to hear opinions from the Working Group, notably those who have implemented RPL? 
> Note that in RPL-11, _the_ preferred parent can actually be a set of parents [2]. 
> Thanks,

I think the correct answer is 1).

Note that the transmission-side damping is RECOMMENDED: a receiver can simply defer decisions, e.g., using hysteresis.

Phil

---

From: JP Vasseur <[email-masked]>
Date: Tue, 3 Aug 2010 08:28:51 +0200

On Aug 2, 2010, at 10:43 PM, Philip Levis wrote:

>
> On Aug 2, 2010, at 1:31 PM, <[email-masked]> <[email-masked] 
> > wrote:
>
>> Hello Phil, all,
>>
>> Our mutual questions and ensuing discussion at the IETF78 meeting  
>> in Maastricht uncovered a different understanding of the way path  
>> costs are propagated in the DODAG.
>>
>> Let's suppose that node X has selected node A as its preferred  
>> parent, and nodes B and C as alternate parents. In addition, let's  
>> suppose node Y is a child for node X.
>>
>> During the life of the network, path costs propagated by DIOs are  
>> likely to fluctuate while the ranks and parenthood relationship are  
>> kept more stable, by only updating them on persistent and  
>> significant changes in costs (see [1]).
>>
>> parent set, as in a distance vector routing protocol. Because B and  
>> C are likely to be used as next-hops in addition to A (low-power  
>> and lossy links come and go), it is right to somehow take them into  
>> account in the cost propagated to Y.
>>
>> * answer 3)
> Note that the transmission-side damping is RECOMMENDED: a receiver  
> can simply defer decisions, e.g., using hysteresis.

Agree with Phil but RPL is open to this. One may define an OF with a  
different policy.

>
> Phil
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