Re: [Roll] MRHOF draft-07 comments

[Federico Consoli <[e-mail]>]

Il 28/03/2012 11.56, Philip Levis ha scritto:
> On Mar 28, 2012, at 12:07 AM, Federico Consoli wrote:
>
>> Hi, I have a doubt about the root's rank with ETX.
>> Assume I have 2 nodes A and B and I use ETX as metric with MRHOF:
>> A is root
>> B is non-root
>> Link between A and B has a cost of 256 (ETX=2)
>>
>> A got rank of 256(2*,4*) and will annunce a path_cost of 0(1*,3*) in its DIO message in its advertised Rank value(6*).
>>
>> B will receive a DIO with rank value = 0 so B will set his rank to 256(7*) and will annunce a path_cost of 256.
>>
>> But now I got 2 nodes with the same rank value. I think it's not a problem with the algorith of DODAG creation but it's not RPL complaint(5*).
> Good catch.
>
> I think the issue here is the MRHOF draft is not clear: A (a root) should advertise a Rank of 256, and B, following 3.3, would advertise a Rank of 256 + MinHopRankIncrease (case 3 of 3.3).
>
> This does get at a more fundamental issue, one where the rules on Rank and the rules on metrics can diverge. I'd argue that the text saying Root nodes set to MIN_PATH_COST should instead relate to MinHopRankIncrease; 3.3 was designed given the RPL specification, but I think the cur_min_path_cost clause is out of sync with it. What do you think?
>
> Phil
Hi, thank you for your reply.

I think that if the root node advertise a Rank of 256 it means that it 
got a path_cost to the worst parent of 256 (1*), so it means that it got 
a ETX of 2 to the worst parent.
It's not RPL complaint(2*).
If instead we suppose that root send packet to itself, it means that the 
root can lost packet (ETX=2) to itself.
IMHO, a way to fix that (still assuming that root send packet to itself) 
is that root could advertise a Rank of 128 (ETX=1) so it got no loss to 
itself.
Anyway I don't like this fix.

I think that a goot fix is that a root set it's rank to 
MinHopRankIncrease and (only root) MUST use the metric container to 
carry ETX=0.

I think it's a pretty significant change in MRHOF but I have other 
ideas. What do you think?




(1*) MRHOF - Section 3.4
"...It then calculates the metric it will advertise in
its metric container. This value is the path cost of the member of
the parent set with the highest path cost..."

(2*) RPL-RFC - Section 8.2.1 - Rules 2
"A DODAG root MUST have a DODAG parent set of size zero"

-- 
Regards
Consoli Federico.