Subject: RE: [Mipshop] Possible Solution to Duplicate Problem?

From: Singh Ajoy-ASINGH1 <[email-masked]>
Date: Sat, 27 Aug 2005 11:32:28 -0500

nope! I have not made such assumption. Your suggested (above) rationale 
is at least wrong because anybody in the know, KNOWS that the L3 is 
preceded by L2. So what you say I assumed is IMPOSSIBLE to happen. The 
fact that you

Ajoy-> What I meant here is that 
MN should prepare for L3 handoff (sending FBU, HI, HACK) once L2 handoff is decision is made. 

Let me comment inline in your model where necessary

If you tightly couple L2/L3

> handoff detection and execution you should not encounter the issue that 
> you are raised. See below an example handoff execution sequence that can be followed:  
> 
>    a. Discover information about neighbors using CARD and other link 
>       layer mechanism. This is done ahead of handoff initiation. 
>    b. Invoke your handoff control algorithm as soon as signal strength of 
>     connected link falls below a given threshold say T1. 

Threshold T1 is for detachment NOT for attachment. The fact that you 
detach from one AP bears NO relevance to where you attach!!!! 

Ajoy-> T1 threshold is used to initiate selection of appropriate AP for 
L2 handoff. At this point you might have list of potential AP(s) or may decide to obtain such list using mechanism such as CARD and by selectively scanning of Neighboring AP(s). This is needed because you do not have to initiate handoff algorithm as long as connected link is in good condition.  

Even 
GPRS/GSM on soft handoff initiation 

Ajoy-> Btw, I have never heard of GSM soft handoff. Soft handoff is used by 
CDMA. Soft-handoff has nothing to do with Mobile/IP handoff. In CDMA network, 
Mobile/IP handoff is performed during hard handoff.  

Even 
GPRS/GSM on soft handoff initiation has got transmission over 3 or more 
BSs since they CANNOT decide with certainty the exact next AP candidate 
until the hysterisis margin settles down. Even then there is compelling 
evidence that the Jacobi iteration for the  matrix iterative technique 
to derive the best SNR _LINGERS_ between BSs with FALSE POSITIVES which 
will only settle when there are LARGE deviations in each matrix iteration.

Ajoy-> These are fine details and should be taken care by handover 
algorithm as described in step a and b. I was giving you high level 
logical steps not fine details of handoff algorithm. 

 b. Using signal strength and other policies capabilities in step a, choose appropriate AP that is the potential candidate for L2 handoff.
> Handoff algorithm should select most appropriate AP at this time. 

here you do not "choose the appropriate AP" as you conviniently 
described. The SNR algo even with hysteresis margin tells you WHEN to 
start looking for a new BS. However

Ajoy-> Again these are fine details of handoff algorithm and should be taken 
by step a and b. 

    1) the search and association occurs AFTER you detach from the 
previous AP NOT BEFORE!!!!

Ajoy-> I agree this is how most existing 802.11 NIC works. But if we really want to enable Fast handoff, you have to do smart scanning. In cellular network scanning is done while you are connected to existing link. 

    2) the indication that you have from mobility/routing neighbourhood 
mappings (i.e. AP/AR mapping) BEFORE the detachment in MULTI-AP overlaps 
that are available concurrently, is NOT indicative of which AP you will 
eventually select during L2 handoff period. 

Both RNC/GPRS and 802.11 do 
exactly this and for GPRS this EXACTLY why their soft handoff does NOT 
have a single EXACT BS candidate for the handoff.

For GSM/GPRS the pilot signals as well as the destined MN data stream in 
soft handoff mode are received SIMULTANEOUSLY until the MN settles into 
the cell CONCRETELY.

Ajoy-> This is where I was disagreeing. You have to plan your system such 
that this assumption is met otherwise you can not do seamless handoff. Even in cellular network once handoff decision is made several messages are exchanged for handoff execution. I am viewing FMIP messages are equivalent to cellular handoff execution signaling. 

For 802.11 we all know that
             a) the MN cannot detect new APs while in active 
transmissions. It must change in monitor/probe mode. I am well aware of 
the efforts about interleaving but they have serious limitations that 
incurs by itself packet loss (which defeats the point) particularly 
during a handoff). Basically they sound nice but under average channel 
load conditions they fail magnificently.

Ajoy-> This is something we should try to optimize. Do you have any solution to fix this? I think this is because of the way 802.11 MAC is designed. In 802.11 there is no notion of control channel so I am not sure if it can fixed the way cellular network has done. 

             b) even if it interleaving was in place the temporal 
location of the MN does NOT guarrantee ANYTHING about the accuracy of 
the selection, particularly when MULTIPLE-APs are around. This is 
because advanced detection of APs (i.e. interleaved) and ultimate 
detachment and L2 handoff (i.e. active scan) cannot be guaranteed to 
yield the SAME AP (i.e. the one selected in advance with the one 
ultimately selected during the scan phase).  I am NOT talking about the 
trivial case of a 2-cell ovelap. This is NOT the average case.

>    c. Using L2->L3 mapping obtained during step a, get the IP address of Access Router Serving the given AP. If AP is being served by multiple ARs and then you can use L3 capability (e.g. load) to select appropriate AR. 
>    c. Initiate L3 handoff (FMIPv6) using the connected link. 
>    d. As soon as L3 handoff is complete, force MN to associate with the AP that was selected during step b.

WHAT???????????????  SINCE WHEN has this been happening in the world of 
wireless handoffs. Do first the L3 handoff and then the L2 one ????
This is certainly either an error on your description or a joke..

Ajoy-> I meant prepare for L3 handoff. 

>    e. If signal strength of connected link falls below a given threshold 
>       and you are unable to complete proactive signaling then associate with 	the selected AP and follow the reactive handoff scenario. 

well this depend completely on which stage of the proactive signalling 
the SNR went below the threshold. And the reactive handoff scenario is a 
MIPv6, i.e. FMIPV6 will have to go through DAD and a few other delays I 
have found and eventually have the samle handoff latency as MIPv6 (so 
there we can remove the "FAST" qualifier from the handoff.

Ajoy-> Ok. Can you share your share your performance data? 

t.

-- 
theo
Nets & Mobile Systems Group
UCL-CS

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From: Theodoros Pagtzis <[email-masked]>
Date: Mon, 29 Aug 2005 00:13:22 +0100

Hi Ajoy,

a few more comments inline.

> MN should prepare for L3 handoff (sending FBU, HI, HACK) once L2 handoff is decision is made. 

IMO this cannot happen. You cannot 'make' first the L2 handoff decision 
because
   1) such decision is SNR based and effected at the link layer not at 
the IP layer. If you are suggesting that FMIPv6 does such thing, this is 
guarranteed to be technology dependent since not all technologies behave 
like that. Also this does not under ANY light qualify as an L2-trigger 
but as an L2-rape (i.e. serious layer violation).

, because you must detach first from the previous AP. You may have an 
indication of possible AP candidates, but the ultimate selection is made 
by SNR at the link layer not by the IP layer AFTER detachment. Even if 
the selection is made before detachment

> Mobile/IP handoff is performed during hard handoff.  

The term GPRSGSM was used to refer to the cellular network.

I am under the impression that GSM is soon to be obsoleted byt 
GPRS/EDGE/UMTS nets. Do you honestly think tha,t Fast Mobile IPv6 will 
be  considered on pure GSM networks???? I hope not otherwise we base our 
design on a technology that is soon to be obsoleted by new standards AND 
implementation on the core net infrastructure!!!! Realistically FMIPv6 
should be expected to be part of the LTE plans not of GSM (I would like 
to see which pure GSM networks will bother with MIPv6/FMIPv6). We design 
for the short/medium/LONG term future, not for tommorrow. Do you really 
think that a GSM network will switch its voice call architecture.

We currently have W-CDMA implementations. We are moving to HSDPA and 
then Enhanced UL. And the best example you mention is the GSM network of 
'99 ???? I definetely not convinced now for the soundness of the mechanism..

> logical steps not fine details of handoff algorithm. 

Such very big "SHOULD" is not very compelling evidence to assure us that 
it will...Would you base your specification/design/product/robustness on 
a "SHOULD" hypothesis???? I hope not..

> Ajoy-> I agree this is how most existing 802.11 NIC works. But if we really want to enable Fast handoff, you have to do smart scanning. 

In cellular network scanning is done while you are connected to existing 
link.

Ajoy, while I am in agreement with the general notion of 'intelligence' 
during an IPv6 handoff, the (at best) abstract 'specification' of 
'smart' scanning for all technologies creates big IFs in the 
specification which in general I am opposed to. When I specify something 
I specify it ROBUSTLY, not with a big IF disclaimer and a pinch of hope.

IMO we either have to be specific on this or ACKNOWLEDGE that this is a 
limitation. Otherwise the specificatin promises ultimately a lot and at 
the moment of truth gives us a big IF to hold on to.

More to the point, when you scan channels as you are connected over the 
existing link and you detect MULTIPLE AP, there is no guarrantee that 
your AP selection BEFORE the T1 SNR threshold for detachment will 
coincide with the actual attachment of the MN with a new AP AFTER 
detachment. This is not just because of SNR, but also a combination of 
traffic load and an increased call blocking or even DROPPING 
probability!! So the intelligence you allude to is significant and 
definetely not tackled by the spec. Abstract intelligence IMO is not 
specification but brainstorm..

> that this assumption is met otherwise you can not do seamless handoff. 
Even in cellular network once handoff decision is made several messages 
are exchanged for handoff execution.
I am viewing FMIP messages are equivalent to cellular handoff execution 
signaling.

Planning is one thing about an assumption, being able to do it is another..

BTW, this is EXACTLY why a GSM network suffers from a SIGNIFICANT 
percentage of ping-pong handoffs. And how does FMIPv6 manage a set of 
cascading FBUs and respective tunnels???? With the reactive handoff 
case, i.e. welcome back good old MIPv6 handoff latency (if it hasn't got 
worse under FMIPv6).

Previous emails have already admitted the limitations of FMIPv6 during 
ping-pong effects. We must look into solutions for these problems 
otherwise, we ran the risk of serious limitations!

> Ajoy-> This is something we should try to optimize. Do you have any solution to fix this? 

yes..don't do it.

I think this is because of the way 802.11 MAC is designed.

exactly!!

In 802.11 there is no notion of control channel so I am not sure if it 
can fixed the way cellular network has done.

And in fact by cellular you allude to GSM, since as you confirmed CDMA 
uses soft-handoffs techniques.

> Ajoy-> I meant prepare for L3 handoff. 

Ok, but I have difficulty in deriving from the (d) the statement 
'prepare for L3 handoff'...but not to worry..

> Ajoy-> Ok. Can you share your share your performance data? 

yes as soon as I am finished with the paper draft.

t.