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Now I've updated the status of everything in my proposal-status list.
241 lines
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Plaintext
241 lines
10 KiB
Plaintext
Filename: 175-automatic-node-promotion.txt
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Title: Automatically promoting Tor clients to nodes
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Author: Steven Murdoch
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Created: 12-Mar-2010
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Status: Rejected
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1. Overview
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This proposal describes how Tor clients could determine when they
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have sufficient bandwidth capacity and are sufficiently reliable to
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become either bridges or Tor relays. When they meet this
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criteria, they will automatically promote themselves, based on user
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preferences. The proposal also defines the new controller messages
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and options which will control this process.
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Note that for the moment, only transitions between client and
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bridge are being considered. Transitions to public relay will
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be considered at a future date, but will use the same
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infrastructure for measuring capacity and reliability.
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2. Motivation and history
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Tor has a growing user-base and one of the major impediments to the
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quality of service offered is the lack of network capacity. This is
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particularly the case for bridges, because these are gradually
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being blocked, and thus no longer of use to people within some
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countries. By automatically promoting Tor clients to bridges, and
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perhaps also to full public relays, this proposal aims to solve
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these problems.
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Only Tor clients which are sufficiently useful should be promoted,
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and the process of determining usefulness should be performed
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without reporting the existence of the client to the central
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authorities. The criteria used for determining usefulness will be
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in terms of bandwidth capacity and uptime, but parameters should be
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specified in the directory consensus. State stored at the client
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should be in no more detail than necessary, to prevent sensitive
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information being recorded.
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3. Design
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3.x Opt-in state model
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Tor can be in one of five node-promotion states:
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- off (O): Currently a client, and will stay as such
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- auto (A): Currently a client, but will consider promotion
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- bridge (B): Currently a bridge, and will stay as such
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- auto-bridge (AB): Currently a bridge, but will consider promotion
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- relay (R): Currently a public relay, and will stay as such
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The state can be fully controlled from the configuration file or
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controller, but the normal state transitions are as follows:
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Any state -> off: User has opted out of node promotion
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Off -> any state: Only permitted with user consent
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Auto -> auto-bridge: Tor has detected that it is sufficiently
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reliable to be a *bridge*
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Auto -> bridge: Tor has detected that it is sufficiently reliable
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to be a *relay*, but the user has chosen to remain a *bridge*
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Auto -> relay: Tor has detected that it is sufficiently reliable
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to be *relay*, and will skip being a *bridge*
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Auto-bridge -> relay: Tor has detected that it is sufficiently
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reliable to be a *relay*
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Note that this model does not support automatic demotion. If this
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is desirable, there should be some memory as to whether the
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previous state was relay, bridge, or auto-bridge. Otherwise the
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user may be prompted to become a relay, although he has opted to
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only be a bridge.
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3.x User interaction policy
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There are a variety of options in how to involve the user into the
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decision as to whether and when to perform node promotion. The
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choice also may be different when Tor is running from Vidalia (and
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thus can readily prompt the user for information), and standalone
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(where Tor can only log messages, which may or may not be read).
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The option requiring minimal user interaction is to automatically
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promote nodes according to reliability, and allow the user to opt
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out, by changing settings in the configuration file or Vidalia user
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interface.
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Alternatively, if a user interface is available, Tor could prompt
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the user when it detects that a transition is available, and allow
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the user to choose which of the available options to select. If
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Vidalia is not available, it still may be possible to solicit an
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email address on install, and contact the operator to ask whether
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a transition to bridge or relay is permitted.
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Finally, Tor could by default not make any transition, and the user
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would need to opt in by stating the maximum level (bridge or
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relay) to which the node may automatically promote itself.
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3.x Performance monitoring model
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To prevent a large number of clients activating as relays, but
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being too unreliable to be useful, clients should measure their
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performance. If this performance meets a parameterized acceptance
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criteria, a client should consider promotion. To measure
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reliability, this proposal adopts a simple user model:
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- A user decides to use Tor at times which follow a Poisson
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distribution
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- At each time, the user will be happy if the bridge chosen has
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adequate bandwidth and is reachable
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- If the chosen bridge is down or slow too many times, the user
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will consider Tor to be bad
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If we additionally assume that the recent history of relay
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performance matches the current performance, we can measure
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reliability by simulating this simple user.
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The following parameters are distributed to clients in the
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directory consensus:
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- min_bandwidth: Minimum self-measured bandwidth for a node to be
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considered useful, in bytes per second
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- check_period: How long, in seconds, to wait between checking
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reachability and bandwidth (on average)
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- num_samples: Number of recent samples to keep
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- num_useful: Minimum number of recent samples where the node was
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reachable and had at least min_bandwidth capacity, for a client
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to consider promoting to a bridge
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A different set of parameters may be used for considering when to
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promote a bridge to a full relay, but this will be the subject of a
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future revision of the proposal.
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3.x Performance monitoring algorithm
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The simulation described above can be implemented as follows:
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Every 60 seconds:
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1. Tor generates a random floating point number x in
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the interval [0, 1).
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2. If x > (1 / (check_period / 60)) GOTO end; otherwise:
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3. Tor sets the value last_check to the current_time (in seconds)
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4. Tor measures reachability
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5. If the client is reachable, Tor measures its bandwidth
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6. If the client is reachable and the bandwidth is >=
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min_bandwidth, the test has succeeded, otherwise it has failed.
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7. Tor adds the test result to the end of a ring-buffer containing
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the last num_samples results: measurement_results
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8. Tor saves last_check and measurements_results to disk
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9. If the length of measurements_results == num_samples and
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the number of successes >= num_useful, Tor should consider
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promotion to a bridge
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end.
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When Tor starts, it must fill in the samples for which it was not
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running. This can only happen once the consensus has downloaded,
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because the value of check_period is needed.
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1. Tor generates a random number y from the Poisson distribution [1]
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with lambda = (current_time - last_check) * (1 / check_period)
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2. Tor sets the value last_check to the current_time (in seconds)
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3. Add y test failures to the ring buffer measurements_results
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4. Tor saves last_check and measurements_results to disk
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In this way, a Tor client will measure its bandwidth and
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reachability every check_period seconds, on average. Provided
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check_period is sufficiently greater than a minute (say, at least an
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hour), the times of check will follow a Poisson distribution. [2]
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While this does require that Tor does record the state of a client
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over time, this does not leak much information. Only a binary
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reachable/non-reachable is stored, and the timing of samples becomes
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increasingly fuzzy as the data becomes less recent.
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On IP address changes, Tor should clear the ring-buffer, because
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from the perspective of users with the old IP address, this node
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might as well be a new one with no history. This policy may change
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once we start allowing the bridge authority to hand out new IP
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addresses given the fingerprint.
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[Perhaps another consensus param? Also, this means we save previous
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IP address in our state file, yes? -RD]
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3.x Bandwidth measurement
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Tor needs to measure its bandwidth to test the usefulness as a
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bridge. A non-intrusive way to do this would be to passively measure
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the peak data transfer rate since the last reachability test. Once
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this exceeds min_bandwidth, Tor can set a flag that this node
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currently has sufficient bandwidth to pass the bandwidth component
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of the upcoming performance measurement.
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For the first version we may simply skip the bandwidth test,
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because the existing reachability test sends 500 kB over several
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circuits, and checks whether the node can transfer at least 50
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kB/s. This is probably good enough for a bridge, so this test
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might be sufficient to record a success in the ring buffer.
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3.x New options
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3.x New controller message
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4. Migration plan
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We should start by setting a high bandwidth and uptime requirement
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in the consensus, so as to avoid overloading the bridge authority
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with too many bridges. Once we are confident our systems can scale,
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the criteria can be gradually shifted down to gain more bridges.
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5. Related proposals
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6. Open questions:
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- What user interaction policy should we take?
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- When (if ever) should we turn a relay into an exit relay?
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- What should the rate limits be for auto-promoted bridges/relays?
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Should we prompt the user for this?
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- Perhaps the bridge authority should tell potential bridges
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whether to enable themselves, by taking into account whether
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their IP address is blocked
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- How do we explain the possible risks of running a bridge/relay
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* Use of bandwidth/congestion
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* Publication of IP address
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* Blocking from IRC (even for non-exit relays)
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- What feedback should we give to bridge relays, to encourage them
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e.g. number of recent users (what about reserve bridges)?
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- Can clients back-off from doing these tests (yes, we should do
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this)
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[1] For algorithms to generate random numbers from the Poisson
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distribution, see: http://en.wikipedia.org/wiki/Poisson_distribution#Generating_Poisson-distributed_random_variables
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[2] "The sample size n should be equal to or larger than 20 and the
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probability of a single success, p, should be smaller than or equal to
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.05. If n >= 100, the approximation is excellent if np is also <= 10."
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http://www.itl.nist.gov/div898/handbook/pmc/section3/pmc331.htm (e-Handbook of Statistical Methods)
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% vim: spell ai et:
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