Subject: [MOBILE-IP] Differences between the two MIPv4 handoff approaches

From: "Karim El-Malki (ERA)" <[email-masked]>
Date: Wed, 4 Oct 2000 20:05:41 +0200

Hello,

I am sending out a list of differences between the two
handoff drafts (draft-calhoun-mobileip-proactive-fa-02 and
draft-elmalki-mobileip-fast-handoffs-03) which Hesham and I
already sent out to the MIPv4 handoffs team. I hope it
will be useful for the WG's handoff discussions.

Regards,
Karim

Proactive Handoffs draft = draft-calhoun-mobileip-proactive-fa-02
Fast Handoffs draft = draft-elmalki-mobileip-fast-handoffs-03

1) The Fast Handoffs draft allows both the network and the
MN to control L3 handoffs just as normal Mobile IP (RFC2002).
The MN performs Mobile IP Registrations and is therefore in
control as to when and with which agent it registers. The
network is also in control since it may decide whether to
send the MN an agent advertisement to trigger the MN's
movement (without which the MN will not perform movement
detection and MIP Registration). The network may also
advertise but reject the MN's Registration and thus again
exercise control. In Proactive handoffs the MN has no control
over the handoff and may only react to the network-controlled
handoff after it has obtained L2 connectivity to the new FA
and starts receiving data from it. Thus the handoff
is forced on the MN and the MN may only react if and
when the new FA decides to advertise.

2) The Fast Handoffs draft allows the MN to register with
the new FA (or GFA) before it moves and before it obtains L2
connectivity to that new FA, thus anticipating the MN's
movement. By interacting with L2 it is possible for the
FA to synchronize such a registration with the cellular
network's mobility such that the new Registration is
completed before the MN is instructed to perform a handoff
at L2. Local HA functionality (GFA) is used to limit the
RTT for the MN's registration. In this way we potentially
eliminate the service disruption caused by L3 handoffs and
limit the MN's service disruption period to that imposed by
the specific L2.

3) The same amount of Registrations/Advertisement are sent/received
by an active MN using Proactive or Fast Handoffs for the support
of real-time services such as VoIP. The main difference is that
in Fast Handoffs the Registration is performed by the MN before
it performs a handoff to the new FA, while in Proactive handoffs
this is done after the MN has performed a handoff to the new
FA and once it is receiving traffic through the new FA.

4) It is not clear in the proactive approach how an intersystem
handoff is handled. For example a handoff occuring between two
different wireless technologies. If the proactive approach is
to be used for handoffs, and if the MN has a choice of two
different systems, it is not possible for the MN to choose which
FA it wants to be connected to (until after the MN has been handed
off to the new FA). Hence a handoff may result in connecting the
MN to a less preferred FA. This is especially true if the FAs
choose not to send agent advertisements for very long periods as
suggested by the draft. This situation will not arise in Fast
Handoffs.

5) The proactive draft states that a FA MAY choose to send an
Agent Advertisement to the MN after the handoff has taken place.
The length of time after which the advertisement is sent is
unclear. If simultaneous bindings are used (which is suggested by
the draft), this will result in long lifetimes for the
simultaneous bindings and consequently an inefficient use of
bandwidth since multiple copies of the same packet are transmitted
to FAs. In Fast Handoffs simultaneous bindings will have short
lifetimes in order to minimize bandwidth consumption.

---

From: Pete McCann <[email-masked]>
Date: Thu, 5 Oct 2000 13:50:15 -0500

Hi,

I wanted to respond to some of the points Karim made, which I hope
will help to clarify the differences between the two drafts:

"Karim El-Malki (ERA)" <[email-masked]> (KE) writes:

KE> Proactive Handoffs draft = draft-calhoun-mobileip-proactive-fa-02
KE> Fast Handoffs draft = draft-elmalki-mobileip-fast-handoffs-03

KE> 1) The Fast Handoffs draft allows both the network and the
KE> MN to control L3 handoffs just as normal Mobile IP (RFC2002).
KE> The MN performs Mobile IP Registrations and is therefore in
KE> control as to when and with which agent it registers. The
KE> network is also in control since it may decide whether to
KE> send the MN an agent advertisement to trigger the MN's
KE> movement (without which the MN will not perform movement
KE> detection and MIP Registration). The network may also
KE> advertise but reject the MN's Registration and thus again
KE> exercise control. In Proactive handoffs the MN has no control
KE> over the handoff and may only react to the network-controlled
KE> handoff after it has obtained L2 connectivity to the new FA
KE> and starts receiving data from it. Thus the handoff
KE> is forced on the MN and the MN may only react if and
KE> when the new FA decides to advertise.

The terms "control" and "force" should be taken with a grain of salt
here.  It is true that the MN, in the Proactive draft, does not
receive an agent advertisement before the handoff, but this is for a
very good reason: we are decoupling the Mobile IP layer-3 handoff from
the underlying layer-2 handoff.  In typical cellular link layers,
there is no time for the Mobile IP protocol (advert/request/response)
to run before the actual handoff must be over and done with and
connectivity to the old FA is lost.

To use Karim's Fast Handoff draft, you need to make sure that the
advert/request/response can run after link-layer power measurements
indicate a likely handoff, but before connectivity to the current
FA is lost.  This imposes very tight real-time requirements on the
MIP client implementation that we were unwilling to impose in the
Proactive draft - my feeling is that this runs counter to the Internet
design philosophy.  You can think of Proactive handoffs as a way to
remove the layer-3 messaging from the critical handoff path; the
network proactively sets up a tunnel from the old point of attachment
to the new point of attachment, and the MN can register there at its
leisure.

KE> 2) The Fast Handoffs draft allows the MN to register with
KE> the new FA (or GFA) before it moves and before it obtains L2
KE> connectivity to that new FA, thus anticipating the MN's
KE> movement. By interacting with L2 it is possible for the
KE> FA to synchronize such a registration with the cellular
KE> network's mobility such that the new Registration is
KE> completed before the MN is instructed to perform a handoff
KE> at L2. Local HA functionality (GFA) is used to limit the
KE> RTT for the MN's registration. In this way we potentially
KE> eliminate the service disruption caused by L3 handoffs and
KE> limit the MN's service disruption period to that imposed by
KE> the specific L2.

Again, this indicates a pretty tight real-time coupling between the
MIP client and the L2 that we wanted to avoid.  Registering after
the L2 handoff removes this dependency.  Also it eliminates the
configuration headaches that are implicit in the Fast Handoffs
scheme: how does the current FA get the advertisement from the
next FA, how does it propagate the registration over multiple
routing hops, and how does the new FA associate the registration
with the soon-to-be-established link-layer connection?

KE> 3) The same amount of Registrations/Advertisement are sent/received
KE> by an active MN using Proactive or Fast Handoffs for the support
KE> of real-time services such as VoIP. The main difference is that
KE> in Fast Handoffs the Registration is performed by the MN before
KE> it performs a handoff to the new FA, while in Proactive handoffs
KE> this is done after the MN has performed a handoff to the new
KE> FA and once it is receiving traffic through the new FA.

Yup.

KE> 4) It is not clear in the proactive approach how an intersystem
KE> handoff is handled. For example a handoff occuring between two
KE> different wireless technologies. If the proactive approach is
KE> to be used for handoffs, and if the MN has a choice of two
KE> different systems, it is not possible for the MN to choose which
KE> FA it wants to be connected to (until after the MN has been handed
KE> off to the new FA). Hence a handoff may result in connecting the
KE> MN to a less preferred FA. This is especially true if the FAs
KE> choose not to send agent advertisements for very long periods as
KE> suggested by the draft. This situation will not arise in Fast
KE> Handoffs.

Inter-system handoffs using the same technology are exactly where
the Proactive approach is intended to be used.  The neighboring FAs
know about each other and have a security association, in the same
way that neighboring Radio Access Networks in today's cellular systems
know about each other (otherwise they would not be able to perform
a hard handoff).  When a handoff from one to another is indicated,
a proactive tunnel is set up and the handoff indication is sent to
the MN.  No involvement from the MN's MIP client is required until
after the real-time handoff is complete.

For inter-system handoffs using different technologies, I don't see
why the network needs to provide any special support over RFC 2002.
The MN can presumably use both technologies simultaneously (otherwise
the handoff cannot possibly be smooth) and can listen for
advertisements on both devices.  When the MN chooses, and without any
special assistance from the network, it may register on the new
technology (let's say, 802.11) and allow its old registration (let's
say, on the cdma2000 network) to expire.  In the meantime, who cares
if the MN was momentarily connected to a new cdma2000 RAN?  The
temporary proactive handoff provides service to the MN while it is
registering on the 802.11 network, and the MN wasn't required to do
anything!

KE> 5) The proactive draft states that a FA MAY choose to send an
KE> Agent Advertisement to the MN after the handoff has taken place.
KE> The length of time after which the advertisement is sent is
KE> unclear. If simultaneous bindings are used (which is suggested by
KE> the draft), this will result in long lifetimes for the
KE> simultaneous bindings and consequently an inefficient use of
KE> bandwidth since multiple copies of the same packet are transmitted
KE> to FAs. In Fast Handoffs simultaneous bindings will have short
KE> lifetimes in order to minimize bandwidth consumption.

Obviously, the network operator must decide how to set these parameters
for optimal network utilization.  If registration is not required for
some time then use of the 'S' bit may not be appropriate.

However, if it is possible for the MN to "ping-pong" between two
different FAs very rapidly, then the simultaneous registrations are
quite useful - multiple copies of packets are exactly what is desired,
on copy transmitted from the old FA (if a link layer connection to the
MN exists there) and another copy from the new FA (again, if a link
layer connection to the MN exists there).  Forcing the MN to send MIP
registrations on each ping-pong event is likely to lead to much
greater strain on the MIP client implementation.

In summary, I think the Fast Handoffs draft has some hidden costs
associated with it (tight L2 coupling, configuration of adverts from
neighbor FAs, getting "remote" registration to work properly) that may
not be immediately obvious on a first read-through.  The Proactive
draft requires some extra support from the network elements but allows
us to use a completely vanilla (non real-time) RFC 2002 MIP client
implementation.  The frequency of advertisements/registrations can be
adjusted for optimum end-to-end performance without regard to the rate
of handoffs in the underlying L2 technology.

-Pete