Proposal idea: Automatically promoting Tor clients to nodes
Steven J. Murdoch
tor+Steven.Murdoch at cl.cam.ac.uk
Wed Mar 31 13:26:01 UTC 2010
I've been working recently on a proposal to increase the number of
bridges in the Tor network. It describes how Tor clients can
automatically become bridges, if they are considered to be
sufficiently reliable and the operator consents.
Comments and suggestions would be appreciated.
The current draft is below, and the latest version can be found here:
Title: Automatically promoting Tor clients to nodes
Author: Steven Murdoch
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
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.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
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
- 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
- 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
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 
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. 
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
 For algorithms to generate random numbers from the Poisson
distribution, see: http://en.wikipedia.org/wiki/Poisson_distribution#Generating_Poisson-distributed_random_variables
 "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)
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