Subject: RE: [Capwap] Proposed Resolution to Issue 45 -Issueoncombiningmessages

From: "Richard Gwee" <[email-masked]>
Date: Tue, 18 Apr 2006 12:58:08 +0800

Hi,

Just like to add in a note. 

In most implementation, the AC does process Echo requests in the sense
that it needs to keep track of which WTP sent it and how often. Thus the
AC is already processing echo requests instead of just acknowledging
back. In reality, scalability should not be an issue.

Regards
Richard

-----Original Message-----
From: Scott G Kelly [mailto:[email-masked]] 
Sent: Monday, April 17, 2006 1:58 AM
To: Richard Gwee
Cc: Saravanan Govindan; [email-masked]
Subject: Re: [Capwap] Proposed Resolution to Issue 45
-Issueoncombiningmessages

Hi Richard,

This is a very difficult conversation to have in email, due to its 
complexity. I'll try to be succinct. I'm trimming much of the 
surrounding context below, as the mixed reply styles (some with '>' 
preceding reply lines, some not) is becoming really confusing. Comments 
inlined below.

Richard Gwee wrote:
> 
> I saw Richard's presentation (and I just reviewed it again in the
> proceedings), but it doesn't actually do the math I asked for.
Instead, 
> it presents a graph that shows two diverging lines, and makes no
attempt
> 
> to explain what is being measured, or why it is reasonable to expect 
> that these values should diverge over time when plotted linearly.
> 
> <Richard> 
> Hi Scott, I am not sure what maths are you really asking for. Is it
> queueing analysis you are referring to? I am not really clear about
your
> maths requirement. Appreciate if you can enlighten me on this aspect.

See response below regarding scenarios...

<trimmed...>

> The second scenario denotes a case in which I have combined both
> statistics and echo messages being exchanged between AC and WTP. Over
a
> long period of time, it is reasonable that these values should diverge
> due to the reduction in overhead when the two messages are combined.
> <Richard-End>

To put this simply, the graph only shows bytes received. There are other

interesting quantities relating to overhead - for example, the 
additional processing requirements this imposes under various 
conditions. You have to think about scaling on the AC side. That would 
be very difficult to account for in a simulation, and depends on the 
implementation.

> An important point: this plot is the result of a simulation. What are 
> the assumptions upon which that simulation is based?
> 
> <Richard>
> The simulation is based on a basic scenario in which I have a model of
> AC connected to a model of WTP and assume no mobile terminals. I
mainly
> focus on the basic interaction between an AC and a WTP.
> <Richard-End>

First, is this really a valid model, with no STA's connected? Adding 
STAs will increase congestion on the LAN side, and perhaps even 
contribute to dropped echo frames.

> I think it probably looks at packet header savings when you eliminate 
> the separate transport headers (which, incidentally, assumes you want
to
> 
> send these statistics at the same interval you want to send echoes),
but
> 
> that doesn't account for the divergence. Maybe Richard can enlighten
us 
> a bit here.
> 
> <Richard>
> I am a bit confused over your reasoning that the packet header savings
> doesn't account for the divergence. My thinking (as well as my results
> have shown) has been that if I have managed to reduce some overhead
due
> to packet header savings, I will definitely see a divergence.
> 
> Appreciate if you can share with me more specifically your reasoning
> behind this. I am interested to know. Thanks.
> <Richard-End>

Your assumptions aren't clear from the graph. Here's a simple summary of

a few ways to look at this:

Scenario 1:
echoes and stats are sent at the same intervals. The redundancy here is 
in L2, L3, L4, and capwap transport headers. In this case, combining the

messages should produce a precisely linear result across time (e.g. 52 
bytes saved per combined message). No divergence.

Scenario 2:
echoes are sent more frequently than stats. Combining these may be worse

if your echo interval is much shorter than your stats interval, because 
now you're sending way more stats than you would have been otherwise, 
for a net loss in efficiency (convergence with crossover, followed by 
negative divergence).

Scenario 3:
stats are sent more frequently than echos. Combining these may better in

some scenarios (looking only at bits on the wire), but this seems like 
an unrealistic approach. Assuming you really would want to do this, 
you'd get divergence with crossover.

What assumptions did you make in your simulation? It is not clear from 
the graph. The only thing I think is clear is that it could not have 
been scenario 1, because this would clearly yield a linear result (i.e. 
parallel lines with no divergence).

> It also seems to assume a perfect processing world - that is, it
ignores
> the increased processing overhead and complexity at either end when
you 
> combine these. And based on Richard's accompanying comments, it also 
> seems to assume a very small statistics payload. As an aside, I think 
> real-world network trace data would be far more enlightening.
> 
> <Richard>
> I do agree that we have to consider many factors such as processing
> overhead and complexity. Unfortunately, my resources are limited. But
to
> be fair, there are few simulations that can really model a real
> scenario. 

...which is why we shouldn't base our decision on simulation results
alone.

> If you feel my simulation is insufficient, I will appreciate
> if you can propose some scenarios for me to simulate and study or can
> point me to some resources where I can gather more enlightening data.
> Thanks.
> <Richard-End> 

Well, this may seem a bit tongue-in-cheek, but perhaps you could try 
running and measuring real traffic? Short of that, it would be nice to 
see results for all three scenarios described above.

------------------
I've trimmed the rest, because I think we're going rather far afield 
here. Dorothy Stanley suggested that we should set aside the question of

combining these questions and first discuss what stats we are interested

in. You have suggested a few, as has Saravan. I am in the process of 
polling folks I work with to see if anything is missing. I would suggest

that all interested vendors do this, and that we try to close on that 
question within a day or two (say, Tuesday?)

I think a critical question we have to answer is, which of the 3 
scenarios I described above is the average case for capwap, i.e. stats 
with every echo, more echoes than stats, or more stats than echoes?

Personally, I think more echoes than stats is the most likely, but I'd 
like to hear other points of view. Only when we know the answer to that 
question can we reasonably discuss the implications of any particular 
optimization strategy.

Scott

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---

From: "Scott G. Kelly" <[email-masked]>
Date: Tue, 18 Apr 2006 10:50:31 -0700 (GMT-07:00)

Just trying to close the loop: I think David's point (i.e. that echoes should only be sent in the absence of other control traffic) is dead on, and that this effectively closes the question as to whether echoes and stats should be combined in the same messages.

Does anyone disagree?

Scott