Hi Maciej,
Maciej Wojciechowski wrote:
You are right, that a control experiment like yours can be very
helpful in getting a better understanding of what is in fact going on.
I'm talking about Marshall's sentence from first post in this thread
"This would be a very encouraging reduction of inter-ISP traffic if it
could be approached in practice by what we are doing here". I think
that if those results could be reproduced it would be great. I'm just
highly doubtful that they can.
We do not intend to overstate our results. We are simply performing a
study intended to
give a first answer to the three questions we present in the introduction:
-How far can we push locality?
-What is, at the scale of a torrent, the reduction of traffic that can
be achieved with locality?
-Can locality significantly deteriorate the peers experience?
I don't know what you mean by reproducibility. If you mean that we will
not find in the Internet
a torrent with the exact same characteristics than the ones we have
evaluated, you are right.
However, our results show that we can push locality further than what
was done up to now
with a great reduction on the inter-ISP traffic. Claiming that this
reduction will be 2 orders of magnitude
on a real torrent, or 1.75 or whatever you want would be pointless. It
will be much better
than what you can obtain with the locality values considered up to now
(around 80%), without a
negative impact on peers download completion time. This is the important
point.
like? Why is it 20kB/s and not 200kB/s or 1037kB/s? Why 100MB and not
257MB or 2057MB? I have never heard of inter ISP link with 2000kB/s
capacity (not mentioning the 40kB/s). I have heard about 10Gbit/s ones
though. I cannot see them anywhere in your work...
Few things that in my opinion are significant and are not taken into
consideration:
- other web activities (like web browsing, gaming, ftp, youtube,...)
of the users
- setting arbitrary max download and max upload speeds
- downloading many torrents at the same time
- vast part of real-world users are hidden behind a NAT
- users with very big symmetric links (e.g. 100Mbit) that are seeding
much more than downloading. Yes, there are such users...
- and many, many more.
Honestly, there are hundreds of factors, many of them very complex. Do
you really want to argue that the simplified model you presented is
good enough to reason about bittorent behavior?
This is exactly the point. There are so many factors that you cannot
test everything.
Either you measure real torrents and traffic, but it is only relevant
for what you have measured at that moment,
and you will never understand this way the impact of some specific
parameters. We did that for BT in another
context (see our IMC'2006 study)
and know that this is just one part of the big picture that allows you
to understand what is going on.
Or, you make controlled experiments varying one parameter at a time. The
question here is how you chose the
fixed parameters and the range of what you vary. This is based on
experience and understanding you got from
in the wild experiments, and related work.
We are quite confident that our choices are relevant and allow us to
show what we wanted to show.
But of course it is always possible to make different choices and to
compare different approaches.
This is why research is great.
I want one thing to be clear: I really like your paper, as a study
work that can help us understand the problem better. But with all due
respect, I simply cannot agree that this is a valid bittorrent model.
Hence, I don't see why those results would give us knowledge about how
the deployed ALTO system would perform in the real-world.
All depends on what you mean by knowledge. Don't attribute to our study
a larger scope than what we described.
Our study simply says that if you increase the locality up to very high
values
then the inter-ISP traffic will be lower without negative impact on
end-users.
Forget about absolute values, they are only there to compare different
experiments in the context of our study.
If you claim that we haven't shown that it makes sense to consider high
locality values, then I have to disagree.
Regards,
Arnaud.
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