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

["Roger K. Alexander" <roger.alexander at ekasystems.com>]

> -----Original Message-----
> From: roll-bounces at ietf.org [mailto:roll-bounces at ietf.org] On Behalf Of
> Richard Kelsey
> Sent: Thursday, November 19, 2009 9:42 AM
> To: JP Vasseur
> Cc: roll at ietf.org
> Subject: Re: [Roll] updating DAO caches (was Re: Something to ADD)
> 
> > 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.
> 
> > 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.

Until a focused P2P mechanism is introduced I would agree that the potential
for P2P traffic flowing through the root will be a consequence that has to
be accepted.

>                                   -Richard Kelsey
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