Re: [Roll] Fwd: I-D Action:draft-dt-roll-rpl-00.txt Q2..4

[Alexandru Petrescu <[e-mail]>]

Pascal Thubert (pthubert) a écrit :
> Hi Alex:
> 
> The links to parents form a DAG. If you include the sibling links it 
> is no more a DAG.

So why talking DAG in the first place?  Do you assume there are no links
between siblings?

> Yet the resulting graph has some properties since there is no 
> orientation that leads deeper;

There should be an orientation that leads deeper too.

> so it is not a general network. The difference between what we are 
> doing and more classical routing is that we REALLY want multipath.

Sorry, I don't understand.  IF we wanted multipath then we preferred
links between siblings and also paths down the DAG, not only up to DAG root.

> So we are ready to forward not only along a classical DAG but also 
> via siblings.

Ok this.

> The decision to pass a given packet to a given sibling is taken on a 
> per packet basis when: - the DAG does not help, none of the parents 
> is usable at this time (e.g. retries failed) - per packet loop 
> avoidance allows that sibling for that packet

Sorry, I don't understand you when you say routing on a per packet 
basis.  All routing is on a per packet basis - it looks at dst, consults 
routing table and then puts the packet on the right interface with the 
right dst MAC address.

> So the whole question is how do we perform per packet loop avoidance.
>  In a very rich world we could actual use a source route header that 
> traces all the hops along a sibling path and try all possibilities. I
> guess we cannot afford that. We could also build a DAG between 
> siblings but again that would reduce our opportunities.

I disagree.  To build a loop-free network there are much more options
than just source route headers and DAGs.  I don't understand the choice
of DAG.

> With the current draft, we resolve the most probable loops and let
> TTL finish the job.

Inefficient DAG.  Unnatural DAG.

> Basically if: - nodes prefer parents over siblings, sibling path 
> should be real short, and mostly 1 hop. - nodes do not give a packet 
> back to a sibling, or there's a tie break there, 1 hop loops would be
> avoided - TTL is decremented aggressively along sibling path (Like 
> minus N instead of minus 1), loops should not cost too much
> 
> The last option is not in the draft yet. It is on the table for 
> discussion. It is a way to adapt the up*down* to IPv6 that has a 
> single counter, hop limit. So we would allow an outward packet to 
> make up to Hop Limit hops along the DAG, but only up to Hop Limit/N 
> hops along siblings, and disallow going outward.

If by this table you mean the table of negotiation of DT I think the 
table should be open to much more options of building routing for 
networks than just DAG and src routing.

Alex