Bandwidth distribution (was: Re: AllowInvalidNodes entry, exit, ... ?)
Sven Anderson
sven at anderson.de
Wed Aug 20 16:19:40 UTC 2008
Hi Mac,
Am 18.08.2008 um 16:43 schrieb macintoshzoom:
> Using "valid nodes" I have noticed too many times mu browsing is
> going to the same exit nodes .... yes fast, but always the same tor
> exit nodes "club".
this is not really a surprise if you look at the distribution of the
bandwidth. I did some graphs for the bandwidth distribution of
yesterday.
As you can see in [1] the distribution of bandwidth over the exit
nodes follows a power-law (aka Pareto, Zipf, heavy/long tail, ...),
like so many other distributions. In the double-logarithmic plot this
is expressed in a linear relation. In this case the linearity starts
between 20 and 30 kB/s. (The bandwidth of the exit nodes is
exponentially binned which results in the equidistant data points.)
These power-law distributions have the well-known characteristic of
many small values and very few big values, also referred to as 90/10
or 80/20 rule. In plot [2] you can see the cumulative distribution
function (CDF) over the ranked exit nodes. As you can see, the 30
biggest exit nodes are holding 50% of the total tor exit bandwidth,
and the 100 biggest hold 70%. While this is still quite moderate it
shows how often you will see the top 30, even if the exit node
selection would only be based on bandwidth. But the "Fast" and
"Stable" flags of course increase this effect.
So there's no conspiracy, it's a natural law.
[1] http://sven.anderson.de/misc/en_bw_dist.pdf
[2] http://sven.anderson.de/misc/en_bw_cdf.pdf
Sven
--
http://sven.anderson.de "Believe those who are seeking the truth.
tel: +49-551-9969285 Doubt those who find it."
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