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Update proposal 161 to reflect mailinglist discussion.
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@ -36,19 +36,27 @@ Status: Open
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if amortized over large stream fetches.
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2. Average Stream Bandwidth Calculation
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3. Average Stream Bandwidth Calculation
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The average stream bandwidths are obtained by dividing the network
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into 3% slices according to advertised node bandwidth, yielding
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about 45 nodes per slice in the current network.
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The average stream bandwidths are obtained by dividing the network into
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slices of 50 nodes each, grouped according to advertised node bandwidth.
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Two hop circuits are built using nodes from the same slice, and a large
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file is downloaded via these circuits. This process is repeated
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several hundred times, and average stream capacities are assigned to
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each node from these results.
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file is downloaded via these circuits. For nodes in the first 15% of the
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network, a 500K file will be used. For nodes in the next 15%, a 250K file
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will be used. For nodes in next 15%, a 100K file will be used. The
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remainder of the nodes will fetch a 75K file.[1]
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This process is repeated 250 times, and average stream capacities are
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assigned to each node from these results.
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In the future, a node generator type can be created to ensure that
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each node is chosen to participate in an equal number of circuits,
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and the selection will continue until every live node is chosen
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to participate in at least 7 circuits.
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3. Ratio Calculation Options
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4. Ratio Calculation Options
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There are two options for deriving the ratios themselves. They can
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be obtained by dividing each nodes' average stream capacity by
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@ -64,7 +72,7 @@ Status: Open
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typically available sooner after a given scan takes place.
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3. Ratio Filtering
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5. Ratio Filtering
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After the base ratios are calculated, a second pass is performed
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to remove any streams with nodes of ratios less than X=0.5 from
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@ -78,7 +86,41 @@ Status: Open
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and one is less than 1.0.
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4. Security implications
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6. Pseudocode for Ratio Calculation Algorithm
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Here is the complete pseudocode for the ratio algorithm:
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Slices = {S | S is 50 nodes of similar consensus capacity}
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for S in Slices:
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while exists node N in S with circ_chosen(N) < 7:
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fetch_slice_file(build_2hop_circuit(N, (exit in S)))
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for N in S:
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BW_measured(N) = MEAN(b | b is bandwidth of a stream through N)
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Bw_stddev(N) = STDDEV(b | b is bandwidth of a stream through N)
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Bw_avg(S) = MEAN(b | b = BW_measured(N) for all N in S)
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Normal_Routers(S) = {N | Bw_measured(N)/Bw_avg(S) > 0.5 }
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for N in S:
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Normal_Streams(N) =
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{stream via N | all nodes in stream not in {Normal_Routers(S)-N}
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and bandwidth > BW_measured(N)-Bw_stddev(N)}
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BW_Norm_measured(N) = MEAN(b | b is a bandwidth of Normal_Streams(N))
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Bw_net_avg(Slices) = MEAN(BW_measured(N) for all N in Slices)
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Bw_Norm_net_avg(Slices) = MEAN(BW_Norm_measured(N) for all N in Slices)
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for N in all Slices:
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Bw_net_ratio(N) = Bw_measured(N)/Bw_net_avg(Slices)
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Bw_Norm_net_ratio(N) = Bw_measured2(N)/Bw_Norm_net_avg(Slices)
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if Bw_net_ratio(N) < 1.0 and Bw_Norm_net_ratio(N) < 1.0:
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ResultRatio(N) = MAX(Bw_net_ratio(N), Bw_Norm_net_ratio(N))
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else if Bw_net_ratio(N) > 1.0 and Bw_Norm_net_ratio(N) > 1.0:
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ResultRatio(N) = MIN(Bw_net_ratio(N), Bw_Norm_net_ratio(N))
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else:
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ResultRatio(N) = MEAN(Bw_net_ratio(N), Bw_Norm_net_ratio(N))
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7. Security implications
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The ratio filtering will deal with cases of sabotage by dropping
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both very slow outliers in stream average calculations, as well
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@ -100,16 +142,24 @@ Status: Open
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does not set us back any in that regard.
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4. Integration with Proposal 160
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8. Integration with Proposal 160
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The final results will be produced for the voting mechanism
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described in Proposal 160 by multiplying the derived ratio by
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the average observed advertised bandwidth during the course of the
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scan. This will produce a new bandwidth value that will be
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output into a file consisting of lines of the form:
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the average published consensus bandwidth during the course of the
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scan, and taking the weighted average with the previous consensus
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bandwidth:
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<node-idhex> SP new_bandwidth NL
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Bw_new = (Bw_current * Alpha + Bw_scan_avg*Bw_ratio)/(Alpha + 1)
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The Alpha parameter is a smoothing parameter intended to prevent
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rapid oscillation between loaded and unloaded conditions.
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This will produce a new bandwidth value that will be output into a
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file consisting of lines of the form:
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node_id=<idhex> SP bw=<Bw_new> NL
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This file can be either copied or rsynced into a directory readable
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by the directory authority.
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