Subject: [Roll] floating DAG question (draft-ietf-roll-rpl-01.txt)

From: Jaudelice de Oliveira <[email-masked]>
Date: Fri, 18 Sep 2009 11:12:14 -0400

In the absence of any LBR or a grounded DAG, is there any provision  
from how two floating DAGs would merge?

Thanks,
Jau.

---

From: Mukul Goyal <[email-masked]>
Date: Fri, 18 Sep 2009 11:24:42 -0500 (CDT)

Jau

Not sure why would one want to merge floating DAGs. Transient floating DAGs ofcourse should not try to merge. Two permanent floating DAGs are advertizing two different popular destinations. So, perhaps, they should not merge either.

Thanks
Mukul

----- Original Message -----
From: "Jaudelice de Oliveira" <[email-masked]>
To: [email-masked]
Cc: "Joydeep Tripathi" <[email-masked]>
Sent: Friday, September 18, 2009 10:12:14 AM GMT -06:00 US/Canada Central
Subject: [Roll] floating DAG question (draft-ietf-roll-rpl-01.txt)

Thanks,
Jau.

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---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Mon, 21 Sep 2009 17:27:15 +0200

Hi Jaudelice:

That would be for instance the use case of nodes isolated in the middle
of nothing, like vibration sensors on a volcano, right? In order to get
any to any communication between those, you need to form one large DAG. 

This is as opposed to the model where you have a backbone infrastructure
of some form and you want to make many shallow DAGs that reach the
nearest exit and that is addressed by OF/OCP 0 in section 5.9 of the
draft 01. 

The short answer is that the OF/OCP determines what happens. If the
OF/OCP is designed such, isolated DAGs should merge in a more or less
orderly fashion to become one larger DAG, and some admin could influence
the selection of the root.

For instance, if you have nodes that are more special than others, you'd
attribute them a preference that is higher. In the end, the node with
the highest preference is the root of the aggregating DAG thus formed. I
had in mind to add the text for an OF/OCP 1 that would do exactly that:

"
OF for OCP1
-----------

OF1 favors the DAG. That is, the Objective Function is designed to use
an administrative preference to select the DAG roots, and build the
largest DAG possible around that sink,whether there is a 'grounded'
topology or not

OF1 selects a preferred parent and a backup next_hop if that's
available. The backup next_hop might be a parent or a sibling. All the
traffic is routed via the preferred parent. When the link conditions do
not let a packet through to the preferred parent, the packet is passed
to the backup next_hop.

The step of rank is 4 for each hop. 

As it scans all the Candidate Neighbors, OF1 keeps the parent that is
the best for the following criteria (in order):

- The interface must be usable and the administrative preference (if
any) applies first. 
- The candidate that would cause to augmenting the rank in the current
DAG is not considered. 
- The candidate that would exposes a DAG with a lesser preference than
self is not considered. 
- A router that is validated as usable is better
- A router that offers connectivity to a grounded DAG is better
- A lesser resulting rank is better
- A DAG for which there is an alternate parent is better. This check is
optional. It is performed by computing the backup next_hop while
assuming that this router won.
- The DAG that was in use already is preferred 
- The router with a better router preference wins
- The preferred parent that was in use already is better.
- If mobile IPv6 is used, a router that exposes a shorter RA-interval is
better
- A router that is fresher (most recent RA) is better

Selection of the backup next_hop
  ---------------------------------

The backup selection is the same as for OF0, since the difference
between the two has to do with the selection of the DAG itself, which is
already done when the backup computation is performed.

"

What do you think?

Pascal

>-----Original Message-----
>From: [email-masked] [mailto:[email-masked]] On Behalf Of
Jaudelice de Oliveira
>Sent: vendredi 18 septembre 2009 17:12
>To: [email-masked]
>Cc: Joydeep Tripathi

---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Mon, 21 Sep 2009 17:53:34 +0200

Hi Mukul.

Maybe and maybe not. 

Like you say, if the DAG is floating due to a reconfigurations, nodes
that can will stay attached to a grounded DAG.
And like you say if the DAG is about reaching something and multiple
DAGs are supported, then there's no point merging.

But if there's support for only one DAG then the only way to maximize
reachability is to merge.

The policy depends on the Objective and thus resides in the OF/OCP...

Pascal

>From: [email-masked] [mailto:[email-masked]] On Behalf Of
Mukul Goyal
>Sent: vendredi 18 septembre 2009 18:25
>To: Jaudelice de Oliveira
>Cc: Joydeep Tripathi; [email-masked]
>Subject: Re: [Roll] floating DAG question (draft-ietf-roll-rpl-01.txt)
>
>Jau
>
>Not sure why would one want to merge floating DAGs. Transient floating
DAGs ofcourse should not try to merge.
>Two permanent floating DAGs are advertizing two different popular
destinations. So, perhaps, they should not
>merge either.

---

From: Emmanuel Baccelli <[email-masked]>
Date: Mon, 21 Sep 2009 18:16:08 +0200

Hi Jaudelice,
In
the scenario that you describe, what kind of trafic patterns are dominant?

Permanent floating DAGs make limited sense if the majority of the traffic is
not multi-point-to-point.

Emmanuel

On Fri, Sep 18, 2009 at 5:12 PM, Jaudelice de Oliveira
<[email-masked]>wrote:

>
> In the absence of any LBR or a grounded DAG, is there any provision from
> how two floating DAGs would merge?
>
> Thanks,
> Jau.
>
>