Re: [Roll] updating DAO caches (was Re: Something to ADD)

[JP Vasseur <jvasseur at cisco.com>]

On Nov 19, 2009, at 3:41 PM, Richard Kelsey wrote:

> > From: JP Vasseur <jvasseur at cisco.com>
> > Date: Thu, 19 Nov 2009 13:18:56 +0100
> >
> > On Nov 19, 2009, at 12:01 PM, Richard Kelsey wrote:
> >
> > JP> From: JP Vasseur <jvasseur at cisco.com>
> > JP> Date: Thu, 19 Nov 2009 11:36:49 +0100
> > JP>
> > JP> I do see several deployment cases where no storing DAO would  
> > lead to
> > JP> extremely sub-optimal paths and even more importantly traffic
> > JP> congestion when getting closer to the root of course.
> >
> > Richard> Can you share these cases with us?  I agree with Jonathan
> > Richard> that storing DAO states will typically not provide much
> > Richard> improvement in P2P routing.
> >
> > Sure. I'll take the example of an inter primary+secondary
> > substation network (could be the smart metering network)
> > that supports traffic for various purposes: meter
> > read-out, DA alarms, ... etc. Thus traffic of various
> > nature: P2MP, MP2P and P2P.
>
> Yes, I think we all agree that reasonably efficient
> P2P is a requirement for many use cases.
>
> The issue isn't whether or not we need good P2P routing.
> The issue is whether or not the DAO mechanism does a good
> enough job to be worth the effort.
>
> RPL ignores P2P when choosing parents.  Getting a good P2P
> route out of RPL is a matter of luck, not design.

First, you can certainly tune the OF to increase the P2P quality.
Second, in the VERY worst non probable case you would transit via
the route.
Last but not least, without DAO how do you send traffic outward ?
Need to wait until you receive a packet, then record route and do
source routing ?

> > There are many such networks
> > where not storing DAO just does not work since all P2P
> > traffic will have to transit via the root (unacceptable
> > delays for alarms) and the traffic around the root will be
> > way too high.
>
> Yes, and in many cases most P2P traffic will have to transit
> via the root even if all nodes store DAOs.  This is a side
> effect of how RPL chooses parents.  Minimizing the lengths
> of the paths to the root maximizes the number of pairwise
> paths that include the root.  If this is unacceptable, then
> we should stop tweaking the DAOs and work on a better
> solution.
>                                  -Richard Kelsey
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