Subject: [MOBILE-IP] Fast Handoff Issues

From: Pete McCann <[email-masked]>
Date: Sun, 23 Jul 2000 14:00:39 -0500

Hi,

I've been catching up on the recent fast handoff threads.  I wanted to
raise a couple of issues about handoff that I think have gone unexamined
here.

First, there is the overhead of link-layer re-establishment.  All of
the proposals I've seen discussed here assume that after a handoff,
the link-layer can be re-established instantaneously.  However,
low-bandwidth links, like cellular interfaces, will make use of
stateful protocols like PPP.  Statefulness is required for header
compression and the negotiation of options like character escaping,
payload compression, and others.  Negotiation at link establishment
time can take several round-trips that could add up to several hundred
milliseconds, depending on latency of the physical layer.

To make handoffs fast, this overhead has to be addressed.  There are
basically 3 options as I see it:

1) Define a new, "stateless" link layer that requires zero negotiation,
   yet nevertheless, meets all of the requirements of a low-bandwidth,
   serial IP connection;
2) Provide some way for the state of the link to "migrate" from one
   point of attachment to the next; or,
3) Allow for data to be tunneled back to the old point of attachment
   for a short period of time until negotiation can be completed at
   the new point of attachment.

In my opinion, (1) would be difficult or impossible to achieve.  (2)
seems to add quite a bit of complexity.  (3) seems the most
reasonable, especially since there is already some amount of
link-layer tunneling present in the 3GPP2 architecture (see the
3gwireless-ext draft).

Second, I would like to see more discussion of inter-domain handoff
issues.  Solving the fast handoff problem within a given (routing)
domain is only part of the problem - we should develop a solution that
is capable of providing fast handoffs from one domain to another, much
in the way that "hard handoff" is designed to work between neighboring
cellular systems run by different operators.  I think it is reasonable
for neighboring BSCs or FAs to have direct relationships, which would
actually be required for doing the kind of "pro-active" handoffs
discussed by Pat & James, that require power measurements of
neighboring cells to be taken.  However, it might not be reasonable
for neighboring systems to share a routing domain or have any GFAs in
common.

Taken together, these two requirements would probably lead to an
architecture most similar to the one in the proactive FA draft, but
it would have a couple of differences:

- Tunneling from previous FA to next FA would be initiated as a side
  effect of the pro-active binding update (in personal communication,
  Pat said this was already possible but I don't think it was mentioned
  directly in the draft).  I think this is discussed in Khalil's draft.

- Data could be tunneled at either layer 3 or layer 2, depending on
  whether the above-mentioned link-layer establishment issues are
  addressed.

- Handoff from one "leaf" FA to another would be more transparent and
  network-controlled, i.e., the MN would not see or need to participate
  in the handoff as long as the GFA didn't change.  Also I think it
  is a mistake to use the "S" bit in this case because you are essentially
  requiring 2x bandwidth for the period of time when the MN is connected
  to both FAs.

I know some of the considerations I've raised are really link-specific
issues, but their impact on the fast handoff mechanism should be
considered.  It may turn out that fast handoff is best addressed in
the appropriate link-layer groups (3GPPs, IEEE) and not the IETF.

Thanks,

-Pete