Subject: RE: [mobile-ip] Problem in FMIPv4

From: [email-masked]
Date: Thu, 9 Jan 2003 17:14:36 -0600

>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?

It is possible that we can override the base MIPv4 spec for MNs that
implement FMIPv4. 

>
>    - 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. 

Better than option 1. 

>
>    - 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.

I prefer this approach as the latency to solicit an FA Advert when the
MN switches to a new link is completely removed.

>    - Other?
>Comments?

---

From: "James Kempf" <[email-masked]>
Date: Fri, 10 Jan 2003 15:52:39 -0800

Raj,

> MN switches to a new link is completely removed.

What do you think of Jon's solution: have the new FA send the Reg Reply to the
old link and also buffer until the MN shows up. If the MN doesn't get it, the MN
solicits a new FAAdvert as soon as it shows up on the new link. The FA sends the
Reg Reply, then the FAAdvert. If the MN gets the Reg Reply, it knows not to
restart the registration, saving the HA RTT.

Note that this would require the new FA to time out the buffered Reg Reply at
some point if it didn't get a SolFAAdvert.

            jak