Subject: [mobile-ip] Problem in FMIPv4

From: "James Kempf" <[email-masked]>
Date: Fri, 3 Jan 2003 14:05:30 -0800

Some of our implementation and measurement experiments of FMIPv4
(draft-ietf-mobileip-lowlatency-handoffs-v4-04.txt) have identified a problem
with Prereg. The protocol says that the Reg Reply from the new Foreign Agent
should be bicast to both the old and new link, so the MN can pick it up on one
or the other depending on when the MN happens to move. This works fine if the
initial Reg Request makes it through on the old link before the link switch
occurs. However, if the link switch cuts off the Reg Request, the MN ends up
waiting on the new link for a Reg Reply that never arrives. The result is a
lengthy wait until the Reg Request timeout defined in the base MIPv4 spec  (1
second, Section 3.6.3 of RFC 3344), at which point the MN fails over to standard
MIP (soliciting for an FA Advert). If the wait didn't occur, the MN could use a
link up trigger on the new link to solicit for an FA right away, considerably
speeding the failover.

Here are some suggested solutions:

    - We can modify the base MIPv4 spec timeout so that it is less than one
second if the MN is using Prereg for handover. Perhaps 100 ms?

    - We can modify the timeout, but make the timeout contingent on a link
switch, so that the MN waits 1 sec or more if the link doesn't switch, but only
waits, say, 100 ms after a link switch. This would allow the FA to send the Reg
Reply immediately after the link switch if the Reg Request succeeded, but also
allow the MN to recover more quickly if not.

    - We can remove the requirement to bicast and have the Reg Reply sent only
to the old link. If the MN doesn't receive the Reg Reply by the time it receives
the link up trigger on the new link, it can solicit a new FA Advert and restart
the Reg Request.

    - Other?

Comments?

---

From: Jonathan Wood <[email-masked]>
Date: Mon, 6 Jan 2003 23:08:40 -0800

Here's a possible solution, assuming the availability of a link-up
L2 trigger:

When the MN receives a link-up trigger, if it has not yet received
a registration reply, it sends a router solicitation.

This handles the case where the reg request or reply has been
lost by immediately commencing standard MIP handover.

If the reg reply has been buffered by the new FA pending arrival
of the MN on link, the MN will receive the reg reply and then
the RA in reply to its RS. Since it has already received the reg
reply, it will not send another reg request upon receipt of the RA.

j