Subject: RE: [mobile-ip] some questions about draft-ietf-mobileip-fast-mip v6-04

From: Michael Thomas <[email-masked]>
Date: Thu, 2 May 2002 06:46:03 -0700 (PDT)

Hesham Soliman (ERA) writes:
 > James,
 > 
 > I rather favor some mechanism to inform the MN after 
 > the move has occured. For example, sending the MN an 
 > ICMP REDIRECT message after the access router has changed, 
 > as Michael has suggested, would do the trick,
 > as it would avoid the problem of the signaling getting cut off
 > prematurely.
 >  
 > => A side note, this is not the way redirects are used. 
 > If there is an indication after the MN moves, then 
 > we should keep it simple and stick to the original
 > MIPv6 design. However, I agree with Rajeev's analysis
 > and suggestions to use the indication before movement.
 > Also, in the MIPv4 draft, we did add a combined method
 > for failure modes where the oFA sends the indication
 > before the MN moves, and if no registration is received
 > till the MN disconnects, the traffic is forwarded to
 > the new FA.
 > This is clearly valid for the case where the network
 > (FA) is aware of the connection/disconnection state
 > of the MN.

So, I agree with Hesham about the way that
redirects actually work. However, back to the
original issue: is there a reason that we can't
have the mobile node pre-approve network based
moves? That would keep with the spirit of the net
(host is in charge), but allow the performance
gains Jim is seeking.

	  Mike

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From: "James Kempf" <[email-masked]>
Date: Thu, 2 May 2002 11:25:52 -0700

Mike,

> (host is in charge), but allow the performance

The question is how far in advance of the handoff is OK?

From the point of view of performance (based on the experiments we've
done with IS-2000 RAN, WLAN, and simulation), the fastest handover
performance is if the access router is informed by the link layer which
access router the mobile will move to, and the access router takes care
of rearranging the routing. The link layer is in the best position to
determine which new access point gives the mobile the best reception
because it has the power measurements, and allowing it to make that
determination and move the mobile without requiring any interaction with
the IP layer is by far the fastest. Making the decision on the access
router is preferable because it eliminates any over the air signaling,
which can get cut off if the mobile moves prematurely. Making the
decision  on the mobile is OK, but signaling at the IP layer prior to
handover risks having the signaling get cut off if the link layer
determines the mobile must be moved quickly to maintain reception. Thus,
allowing the mobile to fix things up after the handover provides more
reliable performance than signaling prior to the handover.

Moving backward in time, the mobile could provide some pre-approval on
the access router enough prior to the handover that the actual handover
sequencing was not affected. Thus, the access router could send a list
of candidate access routers for the next handover, and the mobile could
say which of them it liked and which it didn't. When the time comes for
an actual handover, the access router (if the access router is doing the
choosing) would only select from those access routers on the mobile's
pre-approved list. Or the mobile would choose if the mobile is doing the
choosing. The difficulty with this is that if the link layer decides at
the time of handover that power conditions are best for access points
associated with access router X but access router X is not on the
mobile's list, either the access router must negotiate with the mobile
about the ones the mobile said it didn't want, risking losing the mobile
if power conditions change too rapidly for the negotiation to conclude,
or the mobile will simply be dropped. Similarly, if the mobile is doing
the choosing and access router X is not on its list, the mobile either
better change its mind quickly or risk losing IP service.

Note that this method is particularly well suited to intertechnology
handover, where the mobile must be involved in the handover decision
regardless of the performance implications, because only the mobile
knows which interfaces it currently has active or even installed (for
PCMCIA cards).

Moving backward in time yet further, the mobile could, upon entry to the
wireless network, delegate selection of the next access router to the
network, allowing the network to provide the best performance possible,
or it could indicate that it wants to have a choice. This could be
accomplished by an AAA mechanism in the user profile. For example, when
you sign up for network service, there is a checkbox that allows you to
delegate selection of your access router to the network in return for
better preformance. If you don't check the box, then you get "best
effort" mobile controlled handover, but you get to choose your next
access router on every handoff.

Comments?

                        jak

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From: Michael Thomas <[email-masked]>
Date: Sun, 5 May 2002 12:48:28 -0700 (PDT)

Jim,

I think you're missing one really important
ingredient here: the mobile node wants to be able
to have some say in what the next AR might be
too. In particular, I really, really, want to have
my mobile node be able to, say, tell the cellular
router to create a tunnel to a COA on my home
802.11. In that case, the cellular router knows
nothing at all about my 802.11; it's the MN that
figures this out somehow. In that case, I don't
want it moving me at all as I'm probably
transitioning off their system (or visa versa).

If we factor this scenario in, I think we're
largely in agreement.

	   Mike