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Merge commit 'mikeperry/bandwidth-proposals-final'

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Nick Mathewson 2009-08-09 13:10:06 -07:00
commit 6423091f07
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doc/spec/proposals/161-computing-bandwidth-adjustments.txt
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@ -42,31 +42,25 @@ Status: Open
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. For nodes in the first 15% of the
network, a 500K file will be used. For nodes in the next 15%, a 250K file
will be used. For nodes in next 15%, a 100K file will be used. The
remainder of the nodes will fetch a 75K file.[1]
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
This process is repeated 250 times, and average stream capacities are
assigned to each node from these results.
In the future, a node generator type can be created to ensure that
each node is chosen to participate in an equal number of circuits,
and the selection will continue until every live node is chosen
to participate in at least 7 circuits.
These sizes are based on measurements performed during test scans.
4. Ratio Calculation Options
This process is repeated until each node has been chosen to participate
in at least 5 circuits.
There are two options for deriving the ratios themselves. They can
be obtained by dividing each nodes' average stream capacity by
either the average for the slice, or the average for the network as a
whole.
Dividing by the network-wide average has the advantage that it will
account for issues related to unbalancing between higher vs lower
capacity, such as Steven Murdoch's queuing theory weighting result.
For this reason, we will opt for network-wide averages.
4. Ratio Calculation
The ratios are calculated by dividing each measured value by the
network-wide average.
5. Ratio Filtering
@ -77,10 +71,8 @@ Status: Open
with capacity of one standard deviation below a node's average
are also removed.
The final ratio result will be calculated as the maximum of
these two resulting ratios if both are less than 1.0, the minimum
if both are greater than 1.0, and the mean if one is greater
and one is less than 1.0.
The final ratio result will be greater of the unfiltered ratio
and the filtered ratio.
6. Pseudocode for Ratio Calculation Algorithm
@ -95,11 +87,8 @@ Status: Open
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)
Normal_Routers(S) = {N | Bw_measured(N)/Bw_avg(S) > 0.5 }
for N in S:
Normal_Streams(N) =
{stream via N | all nodes in stream not in {Normal_Routers(S)-N}
and bandwidth > BW_measured(N)-Bw_stddev(N)}
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)
@ -107,14 +96,9 @@ Status: Open
for N in all Slices:
Bw_net_ratio(N) = Bw_measured(N)/Bw_net_avg(Slices)
Bw_Norm_net_ratio(N) = Bw_measured2(N)/Bw_Norm_net_avg(Slices)
Bw_Norm_net_ratio(N) = BW_Norm_measured(N)/Bw_Norm_net_avg(Slices)
if Bw_net_ratio(N) < 1.0 and Bw_Norm_net_ratio(N) < 1.0:
ResultRatio(N) = MAX(Bw_net_ratio(N), Bw_Norm_net_ratio(N))
else if Bw_net_ratio(N) > 1.0 and Bw_Norm_net_ratio(N) > 1.0:
ResultRatio(N) = MIN(Bw_net_ratio(N), Bw_Norm_net_ratio(N))
else:
ResultRatio(N) = MEAN(Bw_net_ratio(N), Bw_Norm_net_ratio(N))
ResultRatio(N) = MAX(Bw_net_ratio(N), Bw_Norm_net_ratio(N))
7. Security implications
@ -126,14 +110,14 @@ Status: Open
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.
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 fetch can also be obscured by using SSL
and periodically changing the large document that is fetched.
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.
@ -148,14 +132,14 @@ Status: Open
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)> NL
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 line that yields a ratio
closest to 1.0 will be chosen in this case.
appear more than once in the file set. The most recently measured
line will be chosen in this case.
9. Integration with Proposal 160
@ -166,10 +150,15 @@ Status: Open
scan, and taking the weighted average with the previous consensus
bandwidth:
Bw_new = (Bw_current * Alpha + Bw_scan_avg*Bw_ratio)/(Alpha + 1)
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.
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:
@ -183,6 +172,3 @@ Status: Open
This file can be either copied or rsynced into a directory readable
by the directory authority.
1. Exact values for each segment are still being determined via
test scans.