Subject: [mobile-ip] Handover to Third?

From: James Kempf <[email-masked]>
Date: Tue, 12 Jun 2001 09:53:04 -0700 (PDT)

I've been looking through the FMIPv6 draft again, and a question came up.

It is possible that the MN may have some traffic on newAR required
to set up the newCoA. It may have to send an F-BU, and an
F-NA or NA. This is in addition to the BU to the HA or hierarchical agent, which 
will require at least another radio link round trip, probably without full 
header compression. So there is some amount of time on newAR during which the MN
is not in full possession of the newCoA, but is in the process of
establishing it.

The question is, what happens if the MN moves to a third AR before
the process of establishing the newCoA is complete?

For example, let's suppose the MN sends the F-BU while on the oldAR
but does not receive a reply before L2 handover to newAR happens. Suppose
further that the MN's location is at a place where three cells butt
up against each other, and that it quickly slips into a third cell at L2,
before the F-BACK arrives. Or suppose the MN is moving quickly through
a small sliver of a cell between two others. This problem is similar
to the ping-pong problem, but with a third cell.

There are two issues here:

1) The newAR may not have enough time to complete the PrRtAdv/F-BU pair
that is required for network assisted handover over the air with MN prior to the 
L2 handover to thirdAR, to say nothing of the RtSolPr/PrRtAdv/F-BU for mobile
assisted. The HI/HACK would probably work, since that is sent over
the backbone and should take no longer than 10 ms, if processing time
on thirdAR is low, and 10 ms is about 1/2 of a radio frame for typical 3G
radio link protocols.

2) The oldAR is now left thinking that MN has moved to newAR. It has
set up a bi-directional tunnel with newAR to assure that MN's packets
get to it, even if they have oCoA as the destination address. It attempts
to send the F-BACK to nCoA, but MN has moved and the F-BACK doesn't get
there, maybe oldAR gets an ICMP message to that effect. What should oldAR do?
How does it distinguish this case from one where the MN has moved and goes into 
dormant mode or is lost? Should newAR set up a second tunnel to thirdAR? 

It seems to me like the draft needs to provide some solution for this
case, as well as for ping-ponging.

		jak

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From: Wolfgang Schoenfeld <[email-masked]>
Date: Wed, 13 Jun 2001 09:54:03 +0200

James,

one may imagine that MN moves (physically or by radio reachability) so fast
that any data forwarded to newAR by whatsoever method
arrives there only after MN has left it.

(This holds for unicast forwarding only.
If data is multicasted to all envisaged ARs,
MN will receive it by at least one of them
- if reachability time is long enough.)

Couldn't it be that the problem you describe is unsolvable in general and
has a solution only under reasonable assumptions on MN's moves,
taking numbers into account as you computed?
These would clearly depend on the (geographical) distribution of ARs.

Wolfgang

-- 
Prof. Dr. Wolfgang Schoenfeld, GMD-IPSI, Dolivostr. 15, D-64293 Darmstadt
Office: +49-6151-869-865 FAX: -6847 Mobile: +49-174-7093455