tor/doc/spec/proposals/161-computing-bandwidth-adjustments.txt

Title: Computing Bandwidth Adjustments
Filename: 161-computing-bandwidth-adjustments.txt
Author: Mike Perry
Created: 12-May-2009
Target: 0.2.2.x
Status: Finished


1. Motivation

  There is high variance in the performance of the Tor network. Despite
  our efforts to balance load evenly across the Tor nodes, some nodes are
  significantly slower and more overloaded than others.

  Proposal 160 describes how we can augment the directory authorities to
  vote on measured bandwidths for routers. This proposal describes what
  goes into the measuring process.


2. Measurement Selection

  The general idea is to determine a load factor representing the ratio
  of the capacity of measured nodes to the rest of the network. This load
  factor could be computed from three potentially relevant statistics:
  circuit failure rates, circuit extend times, or stream capacity.

  Circuit failure rates and circuit extend times appear to be
  non-linearly proportional to node load. We've observed that the same
  nodes when scanned at US nighttime hours (when load is presumably
  lower) exhibit almost no circuit failure, and significantly faster
  extend times than when scanned during the day.

  Stream capacity, however, is much more uniform, even during US
  nighttime hours. Moreover, it is a more intuitive representation of
  node capacity, and also less dependent upon distance and latency
  if amortized over large stream fetches.


3. Average Stream Bandwidth Calculation

  The average stream bandwidths are obtained by dividing the network into
  slices of 50 nodes each, grouped according to advertised node bandwidth.

  Two hop circuits are built using nodes from the same slice, and a large
  file is downloaded via these circuits. The file sizes are set based
  on node percentile rank as follows:
    
     0-10: 2M
     10-20: 1M
     20-30: 512k
     30-50: 256k
     50-100: 128k

  These sizes are based on measurements performed during test scans.

  This process is repeated until each node has been chosen to participate
  in at least 5 circuits.


4. Ratio Calculation

  The ratios are calculated by dividing each measured value by the 
  network-wide average.


5. Ratio Filtering

  After the base ratios are calculated, a second pass is performed
  to remove any streams with nodes of ratios less than X=0.5 from
  the results of other nodes. In addition, all outlying streams
  with capacity of one standard deviation below a node's average
  are also removed.

  The final ratio result will be greater of the unfiltered ratio
  and the filtered ratio.


6. Pseudocode for Ratio Calculation Algorithm

  Here is the complete pseudocode for the ratio algorithm:

    Slices = {S | S is 50 nodes of similar consensus capacity}
    for S in Slices:
      while exists node N in S with circ_chosen(N) < 7:
        fetch_slice_file(build_2hop_circuit(N, (exit in S)))
      for N in S:
        BW_measured(N) = MEAN(b | b is bandwidth of a stream through N)
        Bw_stddev(N) = STDDEV(b | b is bandwidth of a stream through N)
      Bw_avg(S) = MEAN(b | b = BW_measured(N) for all N in S)  
      for N in S:
        Normal_Streams(N) = {stream via N | bandwidth >= BW_measured(N)} 
        BW_Norm_measured(N) =  MEAN(b | b is a bandwidth of Normal_Streams(N))

    Bw_net_avg(Slices) = MEAN(BW_measured(N) for all N in Slices)
    Bw_Norm_net_avg(Slices) = MEAN(BW_Norm_measured(N) for all N in Slices)

    for N in all Slices:
      Bw_net_ratio(N) = Bw_measured(N)/Bw_net_avg(Slices)
      Bw_Norm_net_ratio(N) = BW_Norm_measured(N)/Bw_Norm_net_avg(Slices)

      ResultRatio(N) = MAX(Bw_net_ratio(N), Bw_Norm_net_ratio(N))


7. Security implications

  The ratio filtering will deal with cases of sabotage by dropping
  both very slow outliers in stream average calculations, as well
  as dropping streams that used very slow nodes from the calculation
  of other nodes.

  This scheme will not address nodes that try to game the system by
  providing better service to scanners. The scanners can be detected
  at the entry by IP address, and at the exit by the destination fetch
  IP.

  Measures can be taken to obfuscate and separate the scanners' source
  IP address from the directory authority IP address. For instance,
  scans can happen offsite and the results can be rsynced into the
  authorities. The destination server IP can also change.
 
  Neither of these methods are foolproof, but such nodes can already
  lie about their bandwidth to attract more traffic, so this solution
  does not set us back any in that regard.


8. Parallelization

  Because each slice takes as long as 6 hours to complete, we will want
  to parallelize as much as possible. This will be done by concurrently
  running multiple scanners from each authority to deal with different
  segments of the network. Each scanner piece will continually loop 
  over a portion of the network, outputting files of the form:

   node_id=<idhex> SP strm_bw=<BW_measured(N)> SP 
         filt_bw=<BW_Norm_measured(N)> ns_bw=<CurrentConsensusBw(N)> NL

  The most recent file from each scanner will be periodically gathered 
  by another script that uses them to produce network-wide averages 
  and calculate ratios as per the algorithm in section 6. Because nodes 
  may shift in capacity, they may appear in more than one slice and/or 
  appear more than once in the file set. The most recently measured
  line will be chosen in this case.


9. Integration with Proposal 160

  The final results will be produced for the voting mechanism
  described in Proposal 160 by multiplying the derived ratio by
  the average published consensus bandwidth during the course of the
  scan, and taking the weighted average with the previous consensus
  bandwidth:

     Bw_new = Round((Bw_current * Alpha + Bw_scan_avg*Bw_ratio)/(Alpha + 1))

  The Alpha parameter is a smoothing parameter intended to prevent
  rapid oscillation between loaded and unloaded conditions. It is
  currently fixed at 0.333.

  The Round() step consists of rounding to the 3 most significant figures
  in base10, and then rounding that result to the nearest 1000, with 
  a minimum value of 1000.

  This will produce a new bandwidth value that will be output into a 
  file consisting of lines of the form:

     node_id=<idhex> SP bw=<Bw_new> NL
 
  The first line of the file will contain a timestamp in UNIX time()
  seconds. This will be used by the authority to decide if the 
  measured values are too old to use.
 
  This file can be either copied or rsynced into a directory readable
  by the directory authority.