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on the specifics of recording, storing, and learning circuitbuildtimeout parameters. svn:r16511
113 lines
4.3 KiB
Plaintext
113 lines
4.3 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 at the client, and
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using those statistics to adjust the CircuitBuildTimeout.
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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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Storing Build Times
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Circuit build times will be stored in the circular array
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'circuit_build_times' consisting of uint16_t elements as milliseconds.
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The total size of this array will be based on the number of circuits
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it takes to converge on a good fit of the long term distribution of
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the circuit builds for a fixed link. We do not want this value to be
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too large, because it will make it difficult for clients to adapt to
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moving between different links.
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From our initial observations, this value appears to be on the order
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of 1000, but will be configurable in a #define NCIRCUITS_TO_OBSERVE.
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Long Term Storage
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The long-term storage representation will be implemented by storing a
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histogram with BUILDTIME_BIN_WIDTH millisecond buckets (default 50) when
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writing out the statistics to disk. The format of this histogram on disk
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is yet to be finalized, but it will likely be of the format
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'CircuitBuildTime <bin> <count>'.
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Example:
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CircuitBuildTimeBin 1 100
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CircuitBuildTimeBin 2 50
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...
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Reading the histogram in will entail multiplying each bin by the
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BUILDTIME_BIN_WIDTH and then inserting <count> values into the
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circuit_build_times array each with the value of
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<bin>*BUILDTIME_BIN_WIDTH.
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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.
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We will calculate the parameters for a Pareto distribution
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fitting the data using the estimators at
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http://en.wikipedia.org/wiki/Pareto_distribution#Parameter_estimation.
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The timeout itself will be calculated by solving the CDF for the
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a percentile cutoff BUILDTIME_PERCENT_CUTOFF. This value
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represents the percentage of paths the Tor client will accept out of
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the total number of paths. We have not yet determined a good
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cutoff for this mathematically, but 85% seems a good choice for now.
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From http://en.wikipedia.org/wiki/Pareto_distribution#Definition,
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the calculation we need is pow(BUILDTIME_PERCENT_CUTOFF/100.0, k)/Xm.
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When to Begin Calculation
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The number of circuits to observe (NCIRCUITS_TO_CUTOFF) before
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changing the CircuitBuildTimeout will be tunable via a #define. From
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our measurements, a good value for NCIRCUITS_TO_CUTOFF appears to be
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on the order of 100.
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Dealing with Timeouts
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Timeouts should be counted as the expectation of the region of
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of the Pareto distribution beyond the cutoff. The proposal will
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be updated with this value soon.
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Also, in the event of network failure, the observation mechanism
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should stop collecting timeout data.
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Circuits that timeout will be destroyed, as this indicates one
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or more of their respective nodes are currently overloaded.
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Client Hints
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Some research still needs to be done to provide initial values
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for CircuitBuildTimeout based on values learned from modem
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users, DSL users, Cable Modem users, and dedicated links. A
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radiobutton in Vidalia should eventually be provided that
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sets CircuitBuildTimeout to one of these values and also
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provide the option of purging all learned data, should any exist.
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These values can either be published in the directory, or
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shipped hardcoded for a particular Tor version.
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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. This will eliminate a great deal of the performance
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variation of Tor usage.
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