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

From: "Karim El-Malki (EAB)" <[email-masked]>
Date: Mon, 12 May 2003 20:11:12 +0200

Hi

 > > It seems to limit the gain of using L2 indications.
 > > If you consider the case where you have actually moved 
 > subnet, you need
 > > to send a NS and wait for several seconds before trying to 
 > discover the
 > > new default router. This is slower than if we discarded L2 
 > indications
 > > completely and waited for an RA. I would prefer keeping 
 > the L2 indications
 > > text since it is useful but also allowing the MN to send a 
 > RS immediately
 > > to check whether it has moved. Is it a big problem given 
 > that we already
 > > rate limit responses?
 > 
 > Erik's wording had no real requirements, it is a suggested 
 > implementation.
 > Earlier in this section, it states that this is a generic 
 > non-optimized
 > algorithm, and that we expect that better/faster movement detection
 > algorithms will be proposed and adopted later. This is more to give
 > implementors guidance.

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. Therefore it should facilitate finding the new
default router rather than first checking reachability to the
old one. Instead the proposed informational mechanism seems better
for the case in which the MN has not changed subnet. If the MN sends
an RS first it can at least get an idea whether it has even
unidirectional reachability with the old AR. If it doesn't it can
quickly move on and send it's binding updates instead of having to
wait for the NS procedure before sending the RS.

In essence I would suggest that the MN sends RSs first to get
router info and then (if needed) check whether the old
router is still reachable using NS.

> The method you refer to I believe has 
 > been called
 > "eager cell switching" on this list recently (or is related) 
 > - there are
 > complexities involved with doing that in an optimal manner, 
 > for example
 > consider the case where there are two or more routers on the 
 > link or the MN
 > can hear multiple routers simultaneously (note that hearing 
 > a router's RAs
 > isn't the same as bi-directional reachability confirmation). 
 > That's why
 > elsewhere in this section there is the recommendation that 
 > if you've already
 > discovered a perfectly good bi-directionally reachable 
 > router, you should
 > generally continue to try using it as your default router 
 > even when you
 > receive a RA from a new/different router.

I wasn't considering movement detection algorithms like eager
cell switching in my comment. Running the chosen algorithm would
be the next step after you have all the info on the routers you
can see/reach.

/Karim

---

From: "Ed Remmell" <[email-masked]>
Date: Mon, 12 May 2003 11:32:26 -0700

Karim -

> 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.

> Therefore it should facilitate finding the new
> 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.

To avoid ping-pong'ing (and to avoid the extra overhead associated with
changing the primary CoA), it is important for the MN to verify that the old
router is no longer reachable (either bi-directionally reachable, or
"reachable from", same difference in this case) before switching away from
it. Thus, it is recommended to have some hysteresis w/ regards to preferring
to use the old/current default router rather than switching away from it
whenever you discover a new router. What you are recommending w/ regards to
soliciting for new routers whenever you get any L2 indication of movement
(even if it is just link-up/link-down on the same link, i.e. no change of
link, or a change of access point but no change of access router), in some
deployment scenarios this could result in the MN switching away from a
perfectly good bi-directionally reachable default router when it didn't need
to (RF is a lossy technology, it is possible that the router doesn't receive
your RS or you don't receive its RA, it is possible you get a quick response
from one router and don't wait long enough for all routers on the link to
respond, etc.), and worst case could cause the ping-pong'ing behavior of
switching back and forth quickly between two different routers. For example,
consider what happens when you have multiple routers on the same link, or
you have overlapping RF coverage areas and the MN is able to receive RAs
from multiple routers on different links simultaneously. If the MN has
inadequate L2 triggers to notify it of link movement, at the link-layer if
it is operating at the fringe of signal strength (which can happen if it is
close to a RF null) it could be switching back and forth between different
links/access points, and at the IP layer (w/out L2 triggers) it would appear
to be attached to multiple links simultaneously and to be receiving RAs from
multiple routers on different links. Anyways, in these types of scenarios it
is important to have the hysteresis of preferring to continue to use the old
default router provided it is still confirmed to be bi-directionally
reachable. That why we are suggesting that the MN confirm reachability of
the old default router first, before commencing the L3 handover procedure
which requires it to perform Router Discovery.

If you want us to remove all discussion of L2 triggers, probing, etc. from
this base spec then let's keep discussing this and perhaps we can strip this
down to where it has very little of substance! ;-)

Thanks.
- Ed