Proposal: Computing Bandwidth Adjustments
mikeperry at fscked.org
Wed May 13 02:48:28 UTC 2009
Title: Computing Bandwidth Adjustments
Author: Mike Perry
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.
2. Average Stream Bandwidth Calculation
The average stream bandwidths are obtained by dividing the network
into 3% slices according to advertised node bandwidth, yielding
about 45 nodes per slice in the current network.
Two hop circuits are built using nodes from the same slice, and a large
file is downloaded via these circuits. This process is repeated
several hundred times, and average stream capacities are assigned to
each node from these results.
3. Ratio Calculation Options
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
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.
Dividing by the slice average has the advantage that many scans can
be run in parallel from a single authority, and that results are
typically available sooner after a given scan takes place.
3. 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 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.
4. 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.
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.
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.
4. 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 observed advertised bandwidth during the course of the
scan. This will produce a new bandwidth value that will be
output into a file consisting of lines of the form:
<node-idhex> SP new_bandwidth NL
This file can be either copied or rsynced into a directory readable
by the directory authority.
Mad Computer Scientist
fscked.org evil labs
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