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93 lines
3.7 KiB
Plaintext
93 lines
3.7 KiB
Plaintext
Filename: 151-path-selection-improvements.txt
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Title: Improving Tor Path Selection
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Version:
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Last-Modified:
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Author: Fallon Chen, Mike Perry
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Created: 5-Jul-2008
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Status: Draft
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Overview
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The performance of paths selected can be improved by adjusting the
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CircuitBuildTimeout and avoiding failing guard nodes. This proposal
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describes a method of tracking buildtime statistics, and using those
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statistics to adjust the CircuitBuildTimeout and the number of guards.
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Motivation
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Tor's performance can be improved by excluding those circuits that
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have long buildtimes (and by extension, high latency). For those Tor
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users who require better performance and have lower requirements for
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anonymity, this would be a very useful option to have.
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Implementation
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Learning the CircuitBuildTimeout
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Based on studies of build times, we found that the distribution of
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circuit buildtimes appears to be a Pareto distribution. The number
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of circuits to observe (ncircuits_to_cutoff) before changing the
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CircuitBuildTimeout will be tunable. From out measurements,
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ncircuits_to_cuttoff appears to be on the order of 100.
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In addition, the total number of circuits gathered
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(ncircuits_to_observe) will also be tunable. It is likely that
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ncircuits_to_observe will be somewhere on the order of 1000. The values
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can be represented compactly in Tor in milliseconds as a circular array
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of 16 bit integers. More compact long-term storage representations can
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be implemented by simply storing a histogram with 50 millisecond buckets
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when writing out the statistics to disk.
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Calculating the preferred CircuitBuildTimeout
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Circuits that have longer buildtimes than some x% of the estimated
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CDF of the Pareto distribution will be excluded. x will be tunable
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as well.
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Circuit timeouts
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In the event of a timeout, backoff values should include the 100-x%
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of expected CDF of timeouts. Also, in the event of network failure,
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the observation mechanism should stop collecting timeout data.
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Dropping Failed Guards
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In addition, we have noticed that some entry guards are much more
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failure prone than others. In particular, the circuit failure rates for
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the fastest entry guards was approximately 20-25%, where as slower
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guards exhibit failure rates as high as 45-50%. In [1], it was
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demonstrated that failing guard nodes can deliberately bias path
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selection to improve their success at capturing traffic. For both these
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reasons, failing guards should be avoided.
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We propose increasing the number of entry guards to five, and gathering
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circuit failure statistics on each entry guard. Any guards that exceed
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the average failure rate of all guards by 10% after we have
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gathered ncircuits_to_observe circuits will be replaced.
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Issues
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Impact on anonymity
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Since this follows a Pareto distribution, large reductions on the
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timeout can be achieved without cutting off a great number of the
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total paths. However, hard statistics on which cutoff percentage
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gives optimal performance have not yet been gathered.
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Guard Turnover
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We contend that the risk from failing guards biasing path selection
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outweighs the risk of exposure to larger portions of the network
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for the first hop. Furthermore, from our observations, it appears
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that circuit failure is strongly correlated to node load. Allowing
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clients to migrate away from failing guards should naturally
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rebalance the network, and eventually clients should converge on
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a stable set of reliable guards. It is also likely that once clients
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begin to migrate away from failing guards, their load should go
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down, causing their failure rates to drop as well.
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[1] http://www.crhc.uiuc.edu/~nikita/papers/relmix-ccs07.pdf
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