Subject: Re: [MOBILE-IP] Fast Handoff Issues

From: James Kempf <[email-masked]>
Date: Sun, 23 Jul 2000 13:23:13 -0700

Pete,

Some comments.

>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).
>

In FA assisted handoff, the data continues to be delivered through the old point
of attachment until completion of the network set up at the new point. The
mobile continues to receive data through both points until it breaks off
from the old, when the new point of attachment becomes the
exclusive source of the data. And, as you mention below, tunneling could
be done in FA assisted handoff, though the draft doesn't mention it.

>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.
>

Yes, I've been trying to move discussion in that
direction, but there seems to be a certain resistance to it, for what
I believe are reasons I'll discuss below.

>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.
>

Well, there's an issue you've not included:

- There needs to be some radio-link independent way to transfer radio
quality measurements and radio link context information between the two FAs
at L3.

The reason for this is that the two FAs may not share an L2 RAN connection.
If they do not share this connection, there is no way for radio quality
measurements and radio channel information to be exchanged between the
mobile and the new RAN. If this information cannot be exchanged, the
FA at the old RAN can't judge whether the new RAN is getting good enough
reception to take the mobile and the new RAN can't establish a radio link
with the mobile because it doesn't know what channel the mobile is on.

I think this is a major issue in accommodating inter-domain handoff. There
seems to be a taboo on discussing L2 issues, even if they are accommodated
in a way that is not specific to a particular L2. Unlike 802.11, CDMA and TDMA
need some kind of support for sending L2 information over L3, otherwise,
the new RAN simply can't find the mobile if there is no RAN interconnection.

That said, if there is a RAN connection, there is no need for this information
to be exchanged over L3 because it can be done over (logical) L2.

As I mentioned in my response to Karim, the mobile IP working group may
not be the place to work this out if the taboo on L2 is so strong, but
it *will* have to be worked out somewhere, regardless of the ultimate
fast handoff scheme we adopt, if mobile IP interdomain handoff is to work with
CDMA and TDMA.

                jak

---

From: Pete McCann <[email-masked]>
Date: Mon, 24 Jul 2000 10:17:29 -0500

Hi, James,

>> the appropriate link-layer groups (3GPPs, IEEE) and not the IETF.

JK> Well, there's an issue you've not included:

JK> - There needs to be some radio-link independent way to transfer radio
JK> quality measurements and radio link context information between the two FAs
JK> at L3.

I'm not so sure that there is a need for this.  Just for information,
here's how it works in CDMA today: the BSC, based on pre-provisioned
information, tells the MN to watch the pilot strengths of a number of
neighbor BSCs in a "candidate set."  When some of these measurements
exceed a threshold for a given duration, the MN reports that fact to
the BSC.  The BSC then decides whether to initiate a hard or soft
handoff.  Soft handoff is uninteresting here because it does not
involve a change in the MN's L3 point of attachment.  In hard handoff,
a message is sent to the candidate BSC asking if it has the radio
resources (and other resources) necessary to accept the handoff.  If
so, the handoff is requested and the MN is told on which channels
to re-tune.

To me, this process only makes sense if both BSCs are of the same
technology.  It is theoretically possible for BSCs of different
technologies to run the message flow above, but at each step, the
information must be translated from one system's representation to
another, and the measurements might even be so totally different in
shape that this is not possible.

If you do have two different technologies, such as 802.11 and
cdma2000, then it should be up to the MN to decide when it wants to
hand off.  I do not think we can expect the cdma2000 BSC to get
involved in asking for power measurements from WaveLAN.  The MN
supports both technologies (and there is probably a different pricing
model involved for the two services) so it is in the unique best
position to make a decision.  Also, a fast handoff can be completed
simply by bringing up the WaveLAN before shutting down the cdma2000
link - both interfaces are physically present on the MN so this should
be doable.  No interaction between the two networks is required.

JK> As I mentioned in my response to Karim, the mobile IP working
JK> group may not be the place to work this out if the taboo on L2 is
JK> so strong, but it *will* have to be worked out somewhere,
JK> regardless of the ultimate fast handoff scheme we adopt, if mobile
JK> IP interdomain handoff is to work with CDMA and TDMA.

I guess I was really talking about inter-domain, single technology
handoffs, because it seems to me that the inter-domain, different
technology handoff problem has a simple solution (let the MN decide).
So maybe "proactive FA" only makes sense in the case of homogeneous
radio transmission technology, but I think it can be of great benefit
there, assuming we can solve the "PPP re-establishment" problem.  The
way I think of the proactive FA solution is that there is always an
unshown flow of information between the BSCs attached to each leaf FA
to exchange information and decide whether the handoff should take
place.  It seems to me this flow of information should be left
unspoken in the IETF and defined elsewhere (i.e., in the 3GPPs).

-Pete