Subject: RE: [mobile-ip] newest text for MIPv6 movement detection

From: "Ed Remmell" <[email-masked]>
Date: Mon, 12 May 2003 13:24:33 -0700

Karim -

> Please help clarify, it's late in the day here and maybe I'm
> getting a bit slow!

Or perhaps it is me... ;-)

> If I physically disconnect from Router A
> and connect to Router B and get a L2 indication, independently
> of whether I use Eager or Lazy Cell Switching or
> something else, I have physically moved to another subnet.

Does your MN know that it has moved to a different physical link? Could it
still be on the same link, but just reassociated (link-down/link-up) for
some reason? If your MN knows for sure that it is attached to a different
physical link, and because of your specific deployment scenario you expect
that that also means a different access router, then I'd treat this as an
indication of a L3 handover, in which case you would skip entirely
determining the reachability of the old default router and start performing
Router Discovery.

> The suggested procedure said that I could send an NS to Router A
> and then send RS if I don't get any reply to the NS.

Basically, you get an indication of a L2 handover, and you need to determine
if it is a L3 handover because it may not be. Your MN very likely doesn't
know that you just physically disconnected it from Router A, and reconnected
it to Router B - all it sees are link-up/link-down events. So, it could have
just reassociated back on the same exact link it was originally attached to,
in which case it is not a L3 handover.

> Since, in this
> typical situation I would solve things by simply sending an RS,
> I thought it would be better to suggest doing the RS before NS.

What you are proposing makes some sense, but it requires extra messaging and
timers. How long do you wait to make sure that you've received all RA
responses to your RS? If I remember correctly, they could be delayed by 1.5
seconds for example. That's a long time to wait, if you are trying to
implement fast movement detection. NS/NA does not have that delay associated
with it - in the case where you are still attached to the same link, NS/NA
will completely very quickly. It is only in the case where you move to a
different link and different access router that it can introduce extra
delay, which is why some people are looking into eager cell switching.

> You would run NS if the old router had replied with a RA.

Would you need to at that point? If the old router replied with a RA, can
you skip NS? Or, should you do the NS/RS in parallel at the same time?
Anyways, what you are suggesting is a potential enhancement, it should be
prototyped/etc., does it really need to go into this base spec now? What is
likely to happen is we just remove any mention of L2 indications from the
base spec if there is going to be a lot of disgreement on it. We are just
trying to include some recommendations for implementors, and identify some
potential pitfalls - as little here as possible.

>  > > Therefore it should facilitate finding the new
>  > > default router rather than first checking reachability to the
>  > > old one.
>  >
>  > There is nothing in the current text (even w/ Erik's
>  > addition about probing)
>  > that precludes you from doing it the way you suggest.
>
> The text talked about a current consensus and the specific
> suggestion seemed to me like it could be improved, that's why
> I commented. If others think it is fine pls go ahead.

- Ed

---

From: "Karim El-Malki (EAB)" <[email-masked]>
Date: Mon, 12 May 2003 21:30:22 +0200

> > What I meant is that I would prefer we suggest a mechanism
 > > that is better for the case in which the MN has actually
 > > changed subnet.
 > 
 > And that mechanism will be something more like "eager cell 
 > switching", i.e.
 > change the default router whenever you receive a RA, w/ some 
 > way to confirm
 > bi-directional reachability of the new router before the 
 > switch and also w/
 > avoiding the ping-pong'ing scenario. "Eager cell switching" 
 > won't go into
 > this draft, because it needs more research/work/experience.

getting a bit slow! If I physically disconnect from Router A
and then send RS if I don't get any reply to the NS. Since, in this

Even if there were 2 routers on the same subnet the above would not
make the mip process slower. After sending a RS you could check
reachability with your old default router and keep using it.

Seems to me like Erik's text works well for this second scenario
but doesn't work well for the first scenario.

/Karim