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59 lines
2.2 KiB
Text
59 lines
2.2 KiB
Text
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 the number of guards. This proposal describes
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a method of tracking buildtime statistics, and using those statistics
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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_observe) before changing the
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CircuitBuildTimeout will be tunable. From our preliminary
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measurements, it is likely that ncircuits_to_observe will be
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somewhere on the order of 1000. The values can be represented
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compactly in Tor in milliseconds as a circular array of 16 bit
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integers. More compact long-term storage representations can be
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implemented by simply storing a histogram with 50 millisecond
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buckets 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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Other notes
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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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Issues
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Impact on anonymity
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