Re: [DNA] Couple of points on draft-jinchoi-dna-soln-frame-00.txt

[Erik Nordmark <Erik.Nordmark@sun.com>]

> >> - that the router lifetime should be honored
> >> - that the prefix lifetimes should be honored
> >> - that NUD would probably happen since the NCE is probably stale
> >> - it may or may not be important to redo DAD
> > 
> > 
> > I think we should try to be more concrete on the last item. 

Yes, we need to think about this more, but I think there are
some tricky issues.

> In section 7.1 of draft-narayanan-dna-bcp-00.txt I wrote:
> 
> "...In the case that the host arrives back on the same link in time less
>     than the DAD completion time (minus a packet transmission and
>     propagation time), the host MAY reclaim the address by sending
>     Neighbor Advertisement messages as if another host had attempted DAD
>     while the host was away.  This will prevent DAD completion by another
>     node upon NA reception.
> 
>     If a host has not been present on a link to defend its address, and
>     has been absent for a full DAD delay (minus transmission time) the
>     host MUST undertake the full DAD procedure for each address from that
>     link it wishes to configure [3]."

This assumes that the host knows exactly when it stopped receiving packet
from the link.  Even if we assume a link-down indication from the device
driver, I doubt that such indications arrive immediately - wouldn't the device
driver and NIC wait until it stops hearing from the access point/base station
that is, there would be a timeout of some sort before a 'link-down' would
be indicated?

Tiggering DAD when the host have been disconnected for 1 second
might be overkill from a probabilistic perspective. Recall that DAD doesn't
try to catch all cases of duplicate addresses by design (hence it doesn't
worry about partitioned links healing, and has a limited number of
transmissions).

If you assume a given arrival rate of new hosts onto a link (which can
potentially have duplicate addresses with the existing hosts) then you can
calculate the probability that a duplicate is missed should one of
the existing hosts be disconnected from the link for N seconds.
It might very well that N can be several minutes without any
significant probability of making DAD detect less duplicates than it
does today (taking packet loss into account).

> Of course, I was writing for current practice, and wasn't relying on
> Optimistic DAD.  For the solution framework, I'd suggest doing Opti-DAD.

Yes, but we still need to decide when to trigger opti-DAD.
For opti-DAD it might be less expensive to tigger than with DAD, so
perhaps we can trigger it more often.

  Erik