Re: [DNA] Redoing DAD [Was: Couple of points ondraft-jinchoi-dna-soln-frame-00.txt]

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

> So in many cases there's a lag or no explicit
> indication.
> In devices using upper layer confirmations of
> traffic, the NC entry for the router before
> router discovery will have been updated before
> the nominal link-down time.
> 
> If we were receiving traffic that is...

I don't understand. The Neighbor Cache Entry isn't updated when
data traffic is received.
Are you talking about the NCE being updated based on the NUD algorithm?
The NUD information is extremely course-grained (with a 30 second timer).

And if the host hasn't recently sent any packets that NUD information
for the router's NCE will be even less exact. You can have:
Time 0: host sends packet with the router as next hop
        The router's NCE was already in DELAY state, so a NUD probe is sent
Time 1ms: The NUD probe receives a response. The NCE is marked as REACHABLE.

3 minutes later (during which no packets have been sent using the NCE)
we get a link-down indication and immediately a link-up indication.

So what does the NCE state say about how long we've been unreachable from the
link?

> DAD may be overkill from a probabilistic view...
> 
> I think you mentioned this yesterday to me:
> 
> If we return within timeout for routers'
> NCE's on the current link, the NS/NA state of
> routers will be updated by an unsolicited NA
> on DAD success (while we were absent).
> 
> In that case, ND/RD messages sent to the routers
> will have a different node's link-layer address.

I guess I don't understand how that relates to DAD though.


> > 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.
> 
> That's fair enough.
> 
> We still have a link seizure though.
> 
> Since we can already delay DAD NS for O-1000 ms,
> we may avoid doing opti-DAD signalling at all if
> we can convince our current router to send an RA
> to us at our 'optimistic' address.
>
> If we've got that existing NCE on the router,
> we may receive the RA before the NS is sent.

I'm not sure that works.
Here is a scenario:

1. Host 1 arrives on the network; does DAD; communicates via the router.
   Thus host 1 has a NCE in the router.
2. Host 1 becomes unreachable.
3. Host 2 arrives and has a duplicate address with H1; performs a DAD 
   which succeds. Host 2 does not communicate with/via the router thus
   the NCE for H1 remains on the router.
4. Host 1 becomes reachable again.

In this case verifying that H1 still has an NCE on the router
(by sending a RS with TSLLAO) doesn't mean that H2 doesn't have a duplicate
address.

  Erik