[tor-dev] Guard node security: ways forward (An update from the dev meeting)

Nicholas Hopper hopper at cs.umn.edu
Tue Feb 25 23:04:17 UTC 2014

On Mon, Feb 24, 2014 at 1:10 PM, George Kadianakis <desnacked at riseup.net> wrote:

>   For example, I'm not sure if I'm using the correct bandwidth
>   values. I'm currently using the value in 'w' lines of the consensus
>   ('w Bandwidth=479'). I used to think that this is a unitless number,
>   but looking at dir-spec.txt it seems to be "kilobytes per
>   second". Is this the value I should be using to figure out which
>   guards I should cut-off?

I was also under the impression that these weights are unitless, but
they do seem to have some correlation to advertised average bandwidth.
 For example, if I sort the valid-after 2100UTC consensus by weights,
and look at the 20 routers starting at weights of 1000,2000,
4000,8000,16000,32000, the median average bandwidth advertised by
these nodes are:

weight - median advertised bandwidth
1000  - 795KBps
2000  - 1049KBps
4000  - 5181KBps
8000  - 6474KBps
16000 -12059KBps
32000 - 31457KBps

(For very low weights and very high weights the correlation breaks
down pretty badly, though)

Any how, we would want to use the weights for cutoffs anyways, since
otherwise "just lie to get above the guard threshold" becomes an
interesting attack.

Another thought: we also should investigate how various thresholds
affect the relationship between the cumulative guard weight total and
the total exit weight.

Nicholas Hopper
Associate Professor, Computer Science & Engineering, University of Minnesota
Visiting Research Director, The Tor Project

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