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

[Richard Kelsey <richard.kelsey at ember.com>]

> From: JP Vasseur <jvasseur at cisco.com>
> Date: Thu, 19 Nov 2009 15:49:15 +0100
> 
> 
> On Nov 19, 2009, at 3:41 PM, Richard Kelsey wrote:
> 
> > 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.

I haven't thought about it a lot, but it isn't at all clear
to me how you would do this.

I believe that we can get good P2P routes using RPL as a
base.  Doing so will require more than just coming up with
an appropriate OF.

> Second, in the VERY worst non probable case you would
> transit via the root.

We clearly have very different images of the DAGs that RPL
will produce.  The DAG formation mechanism works to maximize
downward fanout, which in turn maximizes the number of P2P
routes that go through the root even if all nodes store
DAOs.  For example, if the root has N children with roughly
equal subgraphs, then each node can reach only 1/N of the
network without through the root.

> 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?

My point is that your argument in favor of requiring
multiple DAO caches is also an argument that DAO is not
adequate for P2P routing.

You are claiming that we have to cache DAOs in non-root
nodes because otherwise there is too much traffic through
the root.  If there is too much traffic through the root
without the caching, then there is almost certainly too much
even with the caching.  Having nodes cache DAOs cannot be
counted on to greatly reduce the traffic throught the root.

If the DAO mechanism meets the requirements for P2P, then it
meets those requirements with DAOs only cached at the root.

                            -Richard Kelsey