Re: [Roll] MRHOF draft-07 comments

[Philip Levis <[e-mail]>]

On Mar 29, 2012, at 1:18 PM, Federico Consoli wrote:

> 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"
> 

The RPL specification is what says the Rank value a root must advertise.

8.2.2.2 says
   2.  A DODAG root MUST advertise a rank of ROOT_RANK.

To some degree, absolute ETX values do not matter -- what matters is their relative values. If all DODAG Roots advertise a Rank of 256, corresponding to an ETX of 2, then that's what they advertise, and a route with an computed ETX of 5 involves 3 transmissions within the LLN.

I don't think I'm understanding the issue -- could you explain it again?

Phil