tor/doc/spec/proposals/151-path-selection-improvements.txt

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.