[tor-dev] Implications of switching to a single guard node: some conclusions

[Nicholas Hopper]

 On Thu, Mar 13, 2014 at 5:21 PM, George Kadianakis
<desnacked at riseup.net> wrote:
> tl;dr: analysis seems to indicate that switching to one guard node
> might not be catastrophic to the performance of Tor. To improve
> performance some increased guard bandwidth thresholds are proposed
> that seem to help without completely destroying the anonymity of the
> network. Enjoy the therapeutic qualities of the graphs and please read
> the whole post.

This took way longer than I expected it to, but here we go.  In order
to examine the assumption that "higher guard bandwidth == better
client performance", I modified Aaron Johnson's TorPS simulator to
simulate 50K clients in several different cases:
- "3guards": choose 3 guards according to the current algorithm
- "T=250": choose 1 guard, guard flag assigned as current
- "T=1000": choose 1 guard, filtering guards with measured bandwidth <
1000 KB/s [Note: these former guards then have an increased
probability of being selected as middle and/or exit nodes]
- "T=2000": 1 guard, filter guards with measured bandwidth < T
- "T=3000": ibid
- "T=4000": ibid

For each client, the simulator created 600 circuits, and computed the
maximum bandwidth each circuit could handle (the minimum bandwidth of
the guard, middle and exit relays).  This gives us an empirical
distribution on circuit performance for each client.  Then we can ask
questions about the "typical" bandwidth for a client, e.g. the median
circuit performance.  How many clients typically have low-bandwidth
circuits in each case?  Looking at the 5000 (10%) unluckiest clients,
we see the following behavior:

https://www-users.cs.umn.edu/~hopper/guard_threshold_median_bandwidth.png

Essentially, while the current guard threshold does make the
unluckiest clients pretty sad on average, even a 1MB/s threshold means
that on average things are a little bit brighter than the current
situation, and a 2MB/s threshold for the guard flag makes everyone
happier in the typical case.  (And increasing the threshold beyond
2MB/s doesn't really help much)

(This data is based on the consensus documents from 13 February 2014,
chosen pretty much arbitrarily from the most recent month of archived
consensus documents and relay descriptors)

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