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