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Filename: 151-path-selection-improvements.txt
Title: Improving Tor Path Selection
Author: Fallon Chen, Mike Perry
Created: 5-Jul-2008
Status: Implemented
Overview
The performance of paths selected can be improved by adjusting the
CircuitBuildTimeout and avoiding failing guard nodes. This proposal
describes a method of tracking buildtime statistics at the client, and
using those statistics to adjust the CircuitBuildTimeout.
Motivation
Tor's performance can be improved by excluding those circuits that
have long buildtimes (and by extension, high latency). For those Tor
users who require better performance and have lower requirements for
anonymity, this would be a very useful option to have.
Implementation
Storing Build Times
Circuit build times are stored in the circular array
'circuit_build_times' consisting of uint32_t elements as milliseconds.
The total size of this array is based on the number of circuits
it takes to converge on a good fit of the long term distribution of
the circuit builds for a fixed link. We do not want this value to be
too large, because it will make it difficult for clients to adapt to
moving between different links.
From our observations, this value appears to be on the order of 1000,
but is configurable in a #define NCIRCUITS_TO_OBSERVE.
Long Term Storage
The long-term storage representation is implemented by storing a
histogram with BUILDTIME_BIN_WIDTH millisecond buckets (default 50) when
writing out the statistics to disk. The format this takes in the
state file is 'CircuitBuildTime <bin-ms> <count>', with the total
specified as 'TotalBuildTimes <total>'
Example:
TotalBuildTimes 100
CircuitBuildTimeBin 0 50
CircuitBuildTimeBin 50 25
CircuitBuildTimeBin 100 13
...
Reading the histogram in will entail inserting <count> values
into the circuit_build_times array each with the value of
<bin-ms> milliseconds. In order to evenly distribute the values
in the circular array, the Fisher-Yates shuffle will be performed
after reading values from the bins.
Learning the CircuitBuildTimeout
Based on studies of build times, we found that the distribution of
circuit buildtimes appears to be a Pareto distribution.
We will calculate the parameters for a Pareto distribution
fitting the data using the estimators at
http://en.wikipedia.org/wiki/Pareto_distribution#Parameter_estimation.
The timeout itself is calculated by using the Quartile function (the
inverted CDF) to give us the value on the CDF such that
BUILDTIME_PERCENT_CUTOFF (80%) of the mass of the distribution is
below the timeout value.
Thus, we expect that the Tor client will accept the fastest 80% of
the total number of paths on the network.
Detecting Changing Network Conditions
We attempt to detect both network connectivty loss and drastic
changes in the timeout characteristics. Network connectivity loss
is detected by recording a timestamp every time Tor either completes
a TLS connection or receives a cell. If this timestamp is more than
90 seconds in the past, circuit timeouts are no longer counted.
If more than MAX_RECENT_TIMEOUT_RATE (80%) of the past
RECENT_CIRCUITS (20) time out, we assume the network connection
has changed, and we discard all buildtimes history and compute
a new timeout by estimating a new Pareto curve using the
position on the Pareto Quartile function for the ratio of
timeouts.
Testing
After circuit build times, storage, and learning are implemented,
the resulting histogram should be checked for consistency by
verifying it persists across successive Tor invocations where
no circuits are built. In addition, we can also use the existing
buildtime scripts to record build times, and verify that the histogram
the python produces matches that which is output to the state file in Tor,
and verify that the Pareto parameters and cutoff points also match.
Soft timeout vs Hard Timeout
At some point, it may be desirable to change the cutoff from a
single hard cutoff that destroys the circuit to a soft cutoff and
a hard cutoff, where the soft cutoff merely triggers the building
of a new circuit, and the hard cutoff triggers destruction of the
circuit.
Good values for hard and soft cutoffs seem to be 80% and 60%
respectively, but we should eventually justify this with observation.
When to Begin Calculation
The number of circuits to observe (NCIRCUITS_TO_CUTOFF) before
changing the CircuitBuildTimeout will be tunable via a #define. From
our measurements, a good value for NCIRCUITS_TO_CUTOFF appears to be
on the order of 100.
Dealing with Timeouts
Timeouts should be counted as the expectation of the region of
of the Pareto distribution beyond the cutoff. The proposal will
be updated with this value soon.
Also, in the event of network failure, the observation mechanism
should stop collecting timeout data.
Client Hints
Some research still needs to be done to provide initial values
for CircuitBuildTimeout based on values learned from modem
users, DSL users, Cable Modem users, and dedicated links. A
radiobutton in Vidalia should eventually be provided that
sets CircuitBuildTimeout to one of these values and also
provide the option of purging all learned data, should any exist.
These values can either be published in the directory, or
shipped hardcoded for a particular Tor version.
Issues
Impact on anonymity
Since this follows a Pareto distribution, large reductions on the
timeout can be achieved without cutting off a great number of the
total paths. This will eliminate a great deal of the performance
variation of Tor usage.
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