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854 lines
32 KiB
Plaintext
854 lines
32 KiB
Plaintext
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Tor Guard Specification
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Isis Lovecruft
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George Kadianakis
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Ola Bini
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Nick Mathewson
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1. Introduction and motivation
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Tor uses entry guards to prevent an attacker who controls some
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fraction of the network from observing a fraction of every user's
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traffic. If users chose their entries and exits uniformly at
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random from the list of servers every time they build a circuit,
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then an adversary who had (k/N) of the network would deanonymize
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F=(k/N)^2 of all circuits... and after a given user had built C
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circuits, the attacker would see them at least once with
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probability 1-(1-F)^C. With large C, the attacker would get a
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sample of every user's traffic with probability 1.
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To prevent this from happening, Tor clients choose a small number
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of guard nodes (e.g. 3). These guard nodes are the only
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nodes that the client will connect to directly. If they are not
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compromised, the user's paths are not compromised.
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This specification outlines Tor's guard selection algorithm,
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which tries to meet the following goals:
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- Heuristics and algorithms for determining how and which guards
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are chosen should be kept as simple and easy to understand as
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possible.
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- Clients in censored regions or who are behind a fascist
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firewall who connect to the Tor network should not experience
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any significant disadvantage in terms of reachability or
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usability.
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- Tor should make a best attempt at discovering the most
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appropriate behavior, with as little user input and
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configuration as possible.
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- Tor clients should discover usable guards without too much
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delay.
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- Tor clients should resist (to the extent possible) attacks
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that try to force them onto compromised guards.
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2. State instances
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In the algorithm below, we describe a set of persistent and
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non-persistent state variables. These variables should be
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treated as an object, of which multiple instances can exist.
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In particular, we specify the use of three particular instances:
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A. UseBridges
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If UseBridges is set, then we replace the {GUARDS} set in
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[Sec:GUARDS] below with the list of configured
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bridges. We maintain a separate persistent instance of
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{SAMPLED_GUARDS} and {CONFIRMED_GUARDS} and other derived
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values for the UseBridges case.
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In this case, we impose no upper limit on the sample size.
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B. EntryNodes / ExcludeNodes / Reachable*Addresses /
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FascistFirewall / ClientUseIPv4=0
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If one of the above options is set, and UseBridges is not,
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then we compare the fraction of usable guards in the consensus
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to the total number of guards in the consensus.
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If this fraction is less than {MEANINGFUL_RESTRICTION_FRAC},
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we use a separate instance of the state.
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(While Tor is running, we do not change back and forth between
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the separate instance of the state and the default instance
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unless the fraction of usable guards is 5% higher than, or 5%
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lower than, {MEANINGFUL_RESTRICTION_FRAC}. This prevents us
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from flapping back and forth between instances if we happen to
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hit {MEANINGFUL_RESTRICTION_FRAC} exactly.
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If this fraction is less than {EXTREME_RESTRICTION_FRAC}, we use a
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separate instance of the state, and warn the user.
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[TODO: should we have a different instance for each set of heavily
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restricted options?]
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C. Default
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If neither of the above variant-state instances is used,
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we use a default instance.
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3. Circuit Creation, Entry Guard Selection (1000 foot view)
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A circuit in Tor is a path through the network connecting a client to
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its destination. At a high-level, a three-hop exit circuit will look
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like this:
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Client <-> Entry Guard <-> Middle Node <-> Exit Node <-> Destination
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Entry guards are the only nodes which a client will connect to
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directly. Exit relays are the nodes by which traffic exits the
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Tor network in order to connect to an external destination.
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3.1 Path selection
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For any multi-hop circuit, at least one entry guard and middle node(s) are
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required. An exit node is required if traffic will exit the Tor
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network. Depending on its configuration, a relay listed in a
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consensus could be used for any of these roles. However, this
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specification defines how entry guards specifically should be selected and
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managed, as opposed to middle or exit nodes.
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3.1.1 Entry guard selection
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At a high level, a relay listed in a consensus will move through the
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following states in the process from initial selection to eventual
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usage as an entry guard:
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relays listed in consensus
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sampled
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confirmed filtered
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primary usable_filtered
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Relays listed in the latest consensus can be sampled for guard usage
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if they have the "Guard" flag. Sampling is random but weighted by
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bandwidth.
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Once a path is built and a circuit established using this guard, it
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is marked as confirmed. Until this point, guards are first sampled
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and then filtered based on information such as our current
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configuration (see SAMPLED and FILTERED sections) and later marked as
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usable_filtered if the guard is not primary but can be reached.
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It is always preferable to use a primary guard when building a new
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circuit in order to reduce guard churn; only on failure to connect to
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existing primary guards will new guards be used.
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3.1.2 Middle and exit node selection
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Middle nodes are selected at random from relays listed in the
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latest consensus, weighted by bandwidth. Exit nodes are chosen
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similarly but restricted to relays with a sufficiently permissive
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exit policy.
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3.2 Circuit Building
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Once a path is chosen, Tor will use this path to build a new circuit.
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If the circuit is built successfully, Tor will either use it
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immediately, or Tor will wait for a circuit with a more preferred
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guard if there's a good chance that it will be able to make one.
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If the circuit fails in a way that makes us conclude that a guard
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is not reachable, the guard is marked as unreachable, the circuit is
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closed, and waiting circuits are updated.
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4. The algorithm.
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4.0. The guards listed in the current consensus. [Section:GUARDS]
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By {set:GUARDS} we mean the set of all guards in the current
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consensus that are usable for all circuits and directory
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requests. (They must have the flags: Stable, Fast, V2Dir, Guard.)
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**Rationale**
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We require all guards to have the flags that we potentially need
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from any guard, so that all guards are usable for all circuits.
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4.1. The Sampled Guard Set. [Section:SAMPLED]
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We maintain a set, {set:SAMPLED_GUARDS}, that persists across
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invocations of Tor. It is an unordered subset of the nodes that
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we have seen listed as a guard in the consensus at some point.
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For each such guard, we record persistently:
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- {pvar:ADDED_ON_DATE}: The date on which it was added to
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sampled_guards.
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We set this value to a point in the past, using
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RAND(now, {GUARD_LIFETIME}/10). See
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Appendix [RANDOM] below.
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- {pvar:ADDED_BY_VERSION}: The version of Tor that added it to
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sampled_guards.
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- {pvar:IS_LISTED}: Whether it was listed as a usable Guard in
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the _most recent_ consensus we have seen.
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- {pvar:FIRST_UNLISTED_AT}: If IS_LISTED is false, the publication date
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of the earliest consensus in which this guard was listed such that we
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have not seen it listed in any later consensus. Otherwise "None."
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We randomize this to a point in the past, based on
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RAND(added_at_time, {REMOVE_UNLISTED_GUARDS_AFTER} / 5)
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For each guard in {SAMPLED_GUARDS}, we also record this data,
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non-persistently:
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- {tvar:last_tried_connect}: A 'last tried to connect at'
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time. Default 'never'.
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- {tvar:is_reachable}: an "is reachable" tristate, with
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possible values { <state:yes>, <state:no>, <state:maybe> }.
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Default '<maybe>.'
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[Note: "yes" is not strictly necessary, but I'm
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making it distinct from "maybe" anyway, to make our
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logic clearer. A guard is "maybe" reachable if it's
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worth trying. A guard is "yes" reachable if we tried
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it and succeeded.]
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- {tvar:failing_since}: The first time when we failed to
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connect to this guard. Defaults to "never". Reset to
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"never" when we successfully connect to this guard.
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- {tvar:is_pending} A "pending" flag. This indicates that we
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are trying to build an exploratory circuit through the
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guard, and we don't know whether it will succeed.
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We require that {SAMPLED_GUARDS} contain at least
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{MIN_FILTERED_SAMPLE} guards from the consensus (if possible),
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but not more than {MAX_SAMPLE_THRESHOLD} of the number of guards
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in the consensus, and not more than {MAX_SAMPLE_SIZE} in total.
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(But if the maximum would be smaller than {MIN_FILTERED_SAMPLE}, we
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set the maximum at {MIN_FILTERED_SAMPLE}.)
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To add a new guard to {SAMPLED_GUARDS}, pick an entry at random
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from ({GUARDS} - {SAMPLED_GUARDS}), weighted by bandwidth.
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We remove an entry from {SAMPLED_GUARDS} if:
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* We have a live consensus, and {IS_LISTED} is false, and
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{FIRST_UNLISTED_AT} is over {REMOVE_UNLISTED_GUARDS_AFTER}
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days in the past.
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OR
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* We have a live consensus, and {ADDED_ON_DATE} is over
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{GUARD_LIFETIME} ago, *and* {CONFIRMED_ON_DATE} is either
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"never", or over {GUARD_CONFIRMED_MIN_LIFETIME} ago.
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Note that {SAMPLED_GUARDS} does not depend on our configuration.
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It is possible that we can't actually connect to any of these
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guards.
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**Rationale**
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The {SAMPLED_GUARDS} set is meant to limit the total number of
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guards that a client will connect to in a given period. The
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upper limit on its size prevents us from considering too many
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guards.
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The first expiration mechanism is there so that our
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{SAMPLED_GUARDS} list does not accumulate so many dead
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guards that we cannot add new ones.
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The second expiration mechanism makes us rotate our guards slowly
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over time.
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4.2. The Usable Sample [Section:FILTERED]
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We maintain another set, {set:FILTERED_GUARDS}, that does not
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persist. It is derived from:
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- {SAMPLED_GUARDS}
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- our current configuration,
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- the path bias information.
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A guard is a member of {set:FILTERED_GUARDS} if and only if all
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of the following are true:
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- It is a member of {SAMPLED_GUARDS}, with {IS_LISTED} set to
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true.
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- It is not disabled because of path bias issues.
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- It is not disabled because of ReachableAddresses policy,
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the ClientUseIPv4 setting, the ClientUseIPv6 setting,
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the FascistFirewall setting, or some other
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option that prevents using some addresses.
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- It is not disabled because of ExcludeNodes.
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- It is a bridge if UseBridges is true; or it is not a
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bridge if UseBridges is false.
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- Is included in EntryNodes if EntryNodes is set and
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UseBridges is not. (But see 2.B above).
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We have an additional subset, {set:USABLE_FILTERED_GUARDS}, which
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is defined to be the subset of {FILTERED_GUARDS} where
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{is_reachable} is <yes> or <maybe>.
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We try to maintain a requirement that {USABLE_FILTERED_GUARDS}
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contain at least {MIN_FILTERED_SAMPLE} elements:
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Whenever we are going to sample from {USABLE_FILTERED_GUARDS},
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and it contains fewer than {MIN_FILTERED_SAMPLE} elements, we
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add new elements to {SAMPLED_GUARDS} until one of the following
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is true:
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* {USABLE_FILTERED_GUARDS} is large enough,
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OR
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* {SAMPLED_GUARDS} is at its maximum size.
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** Rationale **
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These filters are applied _after_ sampling: if we applied them
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before the sampling, then our sample would reflect the set of
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filtering restrictions that we had in the past.
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4.3. The confirmed-guard list. [Section:CONFIRMED]
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[formerly USED_GUARDS]
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We maintain a persistent ordered list, {list:CONFIRMED_GUARDS}.
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It contains guards that we have used before, in our preference
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order of using them. It is a subset of {SAMPLED_GUARDS}. For
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each guard in this list, we store persistently:
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- {pvar:IDENTITY} Its fingerprint.
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- {pvar:CONFIRMED_ON_DATE} When we added this guard to
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{CONFIRMED_GUARDS}.
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Randomized to a point in the past as RAND(now, {GUARD_LIFETIME}/10).
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We append new members to {CONFIRMED_GUARDS} when we mark a circuit
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built through a guard as "for user traffic."
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Whenever we remove a member from {SAMPLED_GUARDS}, we also remove
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it from {CONFIRMED_GUARDS}.
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[Note: You can also regard the {CONFIRMED_GUARDS} list as a
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total ordering defined over a subset of {SAMPLED_GUARDS}.]
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Definition: we call Guard A "higher priority" than another Guard B
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if, when A and B are both reachable, we would rather use A. We
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define priority as follows:
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* Every guard in {CONFIRMED_GUARDS} has a higher priority
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than every guard not in {CONFIRMED_GUARDS}.
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* Among guards in {CONFIRMED_GUARDS}, the one appearing earlier
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on the {CONFIRMED_GUARDS} list has a higher priority.
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* Among guards that do not appear in {CONFIRMED_GUARDS},
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{is_pending}==true guards have higher priority.
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* Among those, the guard with earlier {last_tried_connect} time
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has higher priority.
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* Finally, among guards that do not appear in
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{CONFIRMED_GUARDS} with {is_pending==false}, all have equal
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priority.
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** Rationale **
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We add elements to this ordering when we have actually used them
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for building a usable circuit. We could mark them at some other
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time (such as when we attempt to connect to them, or when we
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actually connect to them), but this approach keeps us from
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committing to a guard before we actually use it for sensitive
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traffic.
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4.4. The Primary guards [Section:PRIMARY]
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We keep a run-time non-persistent ordered list of
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{list:PRIMARY_GUARDS}. It is a subset of {FILTERED_GUARDS}. It
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contains {N_PRIMARY_GUARDS} elements.
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To compute primary guards, take the ordered intersection of
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{CONFIRMED_GUARDS} and {FILTERED_GUARDS}, and take the first
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{N_PRIMARY_GUARDS} elements. If there are fewer than
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{N_PRIMARY_GUARDS} elements, append additional elements to
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PRIMARY_GUARDS chosen _uniformly_ at random from
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({FILTERED_GUARDS} - {CONFIRMED_GUARDS}).
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Once an element has been added to {PRIMARY_GUARDS}, we do not remove it
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until it is replaced by some element from {CONFIRMED_GUARDS}. Confirmed
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elements always precede unconfirmed ones in the {PRIMARY_GUARDS} list.
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Note that {PRIMARY_GUARDS} do not have to be in
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{USABLE_FILTERED_GUARDS}: they might be unreachable.
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** Rationale **
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These guards are treated differently from other guards. If one of
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them is usable, then we use it right away. For other guards
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{FILTERED_GUARDS}, if it's usable, then before using it we might
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first double-check whether perhaps one of the primary guards is
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usable after all.
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4.5. Retrying guards. [Section:RETRYING]
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(We run this process as frequently as needed. It can be done once
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a second, or just-in-time.)
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If a primary sampled guard's {is_reachable} status is <no>, then
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we decide whether to update its {is_reachable} status to <maybe>
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based on its {last_tried_connect} time, its {failing_since} time,
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and the {PRIMARY_GUARDS_RETRY_SCHED} schedule.
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If a non-primary sampled guard's {is_reachable} status is <no>, then
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we decide whether to update its {is_reachable} status to <maybe>
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based on its {last_tried_connect} time, its {failing_since} time,
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and the {GUARDS_RETRY_SCHED} schedule.
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** Rationale **
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An observation that a guard has been 'unreachable' only lasts for
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a given amount of time, since we can't infer that it's unreachable
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now from the fact that it was unreachable a few minutes ago.
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4.6. Selecting guards for circuits. [Section:SELECTING]
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Every origin circuit is now in one of these states:
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<state:usable_on_completion>,
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<state:usable_if_no_better_guard>,
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<state:waiting_for_better_guard>, or
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<state:complete>.
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You may only attach streams to <complete> circuits.
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(Additionally, you may only send RENDEZVOUS cells, ESTABLISH_INTRO
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cells, and INTRODUCE cells on <complete> circuits.)
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The per-circuit state machine is:
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New circuits are <usable_on_completion> or
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<usable_if_no_better_guard>.
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A <usable_on_completion> circuit may become <complete>, or may
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fail.
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A <usable_if_no_better_guard> circuit may become
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<usable_on_completion>; may become <waiting_for_better_guard>; or may
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fail.
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A <waiting_for_better_guard> circuit will become <complete>, or will
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be closed, or will fail.
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A <complete> circuit remains <complete> until it fails or is
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closed.
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Each of these transitions is described below.
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We keep, as global transient state:
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* {tvar:last_time_on_internet} -- the last time at which we
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successfully used a circuit or connected to a guard. At
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startup we set this to "infinitely far in the past."
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When we want to build a circuit, and we need to pick a guard:
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* If any entry in PRIMARY_GUARDS has {is_reachable} status of
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<maybe> or <yes>, return one of the first
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{NUM_USABLE_PRIMARY_GUARDS} or
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{NUM_USABLE_PRIMARY_DIRECTORY_GUARDS} such guards, chosen
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uniformly at random. The circuit is <usable_on_completion>.
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[Note: We do not use {is_pending} on primary guards, since we
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are willing to try to build multiple circuits through them
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before we know for sure whether they work, and since we will
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not use any non-primary guards until we are sure that the
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primary guards are all down. (XX is this good?)]
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* Otherwise, if the ordered intersection of {CONFIRMED_GUARDS}
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and {USABLE_FILTERED_GUARDS} is nonempty, return the first
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entry in that intersection that has {is_pending} set to
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false. Set its value of {is_pending} to true. The circuit
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is now <usable_if_no_better_guard>. (If all entries have
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{is_pending} true, pick the first one.)
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* Otherwise, if there is no such entry, select a member at
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random from {USABLE_FILTERED_GUARDS}. Set its {is_pending}
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field to true. The circuit is <usable_if_no_better_guard>.
|
|
|
|
* Otherwise, if USABLE_FILTERED_GUARDS is empty, we have exhausted
|
|
all the sampled guards. In this case we proceed by marking all guards
|
|
as <maybe> reachable so that we can keep on trying circuits.
|
|
|
|
Whenever we select a guard for a new circuit attempt, we update the
|
|
{last_tried_connect} time for the guard to 'now.'
|
|
|
|
In some cases (for example, when we need a certain directory feature,
|
|
or when we need to avoid using a certain exit as a guard), we need to
|
|
restrict the guards that we use for a single circuit. When this happens, we
|
|
remember the restrictions that applied when choosing the guard for
|
|
that circuit, since we will need them later (see [UPDATE_WAITING].).
|
|
|
|
** Rationale **
|
|
|
|
We're getting to the core of the algorithm here. Our main goals are to
|
|
make sure that
|
|
|
|
1. If it's possible to use a primary guard, we do.
|
|
2. We probably use the first primary guard.
|
|
|
|
So we only try non-primary guards if we're pretty sure that all
|
|
the primary guards are down, and we only try a given primary guard
|
|
if the earlier primary guards seem down.
|
|
|
|
When we _do_ try non-primary guards, however, we only build one
|
|
circuit through each, to give it a chance to succeed or fail. If
|
|
ever such a circuit succeeds, we don't use it until we're pretty
|
|
sure that it's the best guard we're getting. (see below).
|
|
|
|
[XXX timeout.]
|
|
|
|
4.7. When a circuit fails. [Section:ON_FAIL]
|
|
|
|
When a circuit fails in a way that makes us conclude that a guard
|
|
is not reachable, we take the following steps:
|
|
|
|
* Set the guard's {is_reachable} status to <no>. If it had
|
|
{is_pending} set to true, we make it non-pending.
|
|
|
|
* Close the circuit, of course. (This removes it from
|
|
consideration by the algorithm in [UPDATE_WAITING].)
|
|
|
|
* Update the list of waiting circuits. (See [UPDATE_WAITING]
|
|
below.)
|
|
|
|
[Note: the existing Tor logic will cause us to create more
|
|
circuits in response to some of these steps; and also see
|
|
[ON_CONSENSUS].]
|
|
|
|
** Rationale **
|
|
|
|
See [SELECTING] above for rationale.
|
|
|
|
4.8. When a circuit succeeds [Section:ON_SUCCESS]
|
|
|
|
When a circuit succeeds in a way that makes us conclude that a
|
|
guard _was_ reachable, we take these steps:
|
|
|
|
* We set its {is_reachable} status to <yes>.
|
|
* We set its {failing_since} to "never".
|
|
* If the guard was {is_pending}, we clear the {is_pending} flag.
|
|
* If the guard was not a member of {CONFIRMED_GUARDS}, we add
|
|
it to the end of {CONFIRMED_GUARDS}.
|
|
|
|
* If this circuit was <usable_on_completion>, this circuit is
|
|
now <complete>. You may attach streams to this circuit,
|
|
and use it for hidden services.
|
|
|
|
* If this circuit was <usable_if_no_better_guard>, it is now
|
|
<waiting_for_better_guard>. You may not yet attach streams to it.
|
|
Then check whether the {last_time_on_internet} is more than
|
|
{INTERNET_LIKELY_DOWN_INTERVAL} seconds ago:
|
|
|
|
* If it is, then mark all {PRIMARY_GUARDS} as "maybe"
|
|
reachable.
|
|
|
|
* If it is not, update the list of waiting circuits. (See
|
|
[UPDATE_WAITING] below)
|
|
|
|
[Note: the existing Tor logic will cause us to create more
|
|
circuits in response to some of these steps; and see
|
|
[ON_CONSENSUS].]
|
|
|
|
** Rationale **
|
|
|
|
See [SELECTING] above for rationale.
|
|
|
|
4.9. Updating the list of waiting circuits [Section:UPDATE_WAITING]
|
|
|
|
We run this procedure whenever it's possible that a
|
|
<waiting_for_better_guard> circuit might be ready to be called
|
|
<complete>.
|
|
|
|
* If any circuit C1 is <waiting_for_better_guard>, AND:
|
|
* All primary guards have reachable status of <no>.
|
|
* There is no circuit C2 that "blocks" C1.
|
|
Then, upgrade C1 to <complete>.
|
|
|
|
Definition: In the algorithm above, C2 "blocks" C1 if:
|
|
* C2 obeys all the restrictions that C1 had to obey, AND
|
|
* C2 has higher priority than C1, AND
|
|
* Either C2 is <complete>, or C2 is <waiting_for_better_guard>,
|
|
or C2 has been <usable_if_no_better_guard> for no more than
|
|
{NONPRIMARY_GUARD_CONNECT_TIMEOUT} seconds.
|
|
|
|
We run this procedure periodically:
|
|
|
|
* If any circuit stays in <waiting_for_better_guard>
|
|
for more than {NONPRIMARY_GUARD_IDLE_TIMEOUT} seconds,
|
|
time it out.
|
|
|
|
**Rationale**
|
|
|
|
If we open a connection to a guard, we might want to use it
|
|
immediately (if we're sure that it's the best we can do), or we
|
|
might want to wait a little while to see if some other circuit
|
|
which we like better will finish.
|
|
|
|
|
|
When we mark a circuit <complete>, we don't close the
|
|
lower-priority circuits immediately: we might decide to use
|
|
them after all if the <complete> circuit goes down before
|
|
{NONPRIMARY_GUARD_IDLE_TIMEOUT} seconds.
|
|
|
|
4.10. Whenever we get a new consensus. [Section:ON_CONSENSUS]
|
|
|
|
We update {GUARDS}.
|
|
|
|
For every guard in {SAMPLED_GUARDS}, we update {IS_LISTED} and
|
|
{FIRST_UNLISTED_AT}.
|
|
|
|
[**] We remove entries from {SAMPLED_GUARDS} if appropriate,
|
|
according to the sampled-guards expiration rules. If they were
|
|
in {CONFIRMED_GUARDS}, we also remove them from
|
|
{CONFIRMED_GUARDS}.
|
|
|
|
We recompute {FILTERED_GUARDS}, and everything that derives from
|
|
it, including {USABLE_FILTERED_GUARDS}, and {PRIMARY_GUARDS}.
|
|
|
|
(Whenever one of the configuration options that affects the
|
|
filter is updated, we repeat the process above, starting at the
|
|
[**] line.)
|
|
|
|
4.11. Deciding whether to generate a new circuit.
|
|
[Section:NEW_CIRCUIT_NEEDED]
|
|
|
|
We generate a new circuit when we don't have
|
|
enough circuits either built or in-progress to handle a given
|
|
stream, or an expected stream.
|
|
|
|
For the purpose of this rule, we say that <waiting_for_better_guard>
|
|
circuits are neither built nor in-progress; that <complete>
|
|
circuits are built; and that the other states are in-progress.
|
|
|
|
4.12. When we are missing descriptors.
|
|
[Section:MISSING_DESCRIPTORS]
|
|
|
|
We need either a router descriptor or a microdescriptor in order
|
|
to build a circuit through a guard. If we do not have such a
|
|
descriptor for a guard, we can still use the guard for one-hop
|
|
directory fetches, but not for longer circuits.
|
|
|
|
(Also, when we are missing descriptors for our first
|
|
{NUM_USABLE_PRIMARY_GUARDS} primary guards, we don't build
|
|
circuits at all until we have fetched them.)
|
|
|
|
A. Appendices
|
|
|
|
A.0. Acknowledgements
|
|
|
|
This research was supported in part by NSF grants CNS-1111539,
|
|
CNS-1314637, CNS-1526306, CNS-1619454, and CNS-1640548.
|
|
|
|
A.1. Parameters with suggested values. [Section:PARAM_VALS]
|
|
|
|
(All suggested values chosen arbitrarily)
|
|
|
|
{param:MAX_SAMPLE_THRESHOLD} -- 20%
|
|
|
|
{param:MAX_SAMPLE_SIZE} -- 60
|
|
|
|
{param:GUARD_LIFETIME} -- 120 days
|
|
|
|
{param:REMOVE_UNLISTED_GUARDS_AFTER} -- 20 days
|
|
[previously ENTRY_GUARD_REMOVE_AFTER]
|
|
|
|
{param:MIN_FILTERED_SAMPLE} -- 20
|
|
|
|
{param:N_PRIMARY_GUARDS} -- 3
|
|
|
|
{param:PRIMARY_GUARDS_RETRY_SCHED}
|
|
-- every 30 minutes for the first 6 hours.
|
|
-- every 2 hours for the next 3.75 days.
|
|
-- every 4 hours for the next 3 days.
|
|
-- every 9 hours thereafter.
|
|
|
|
{param:GUARDS_RETRY_SCHED} -- 1 hour
|
|
-- every hour for the first 6 hours.
|
|
-- every 4 hours for the next 3.75 days.
|
|
-- every 18 hours for the next 3 days.
|
|
-- every 36 hours thereafter.
|
|
|
|
{param:INTERNET_LIKELY_DOWN_INTERVAL} -- 10 minutes
|
|
|
|
{param:NONPRIMARY_GUARD_CONNECT_TIMEOUT} -- 15 seconds
|
|
|
|
{param:NONPRIMARY_GUARD_IDLE_TIMEOUT} -- 10 minutes
|
|
|
|
{param:MEANINGFUL_RESTRICTION_FRAC} -- .2
|
|
|
|
{param:EXTREME_RESTRICTION_FRAC} -- .01
|
|
|
|
{param:GUARD_CONFIRMED_MIN_LIFETIME} -- 60 days
|
|
|
|
{param:NUM_USABLE_PRIMARY_GUARDS} -- 1
|
|
|
|
{param:NUM_USABLE_PRIMARY_DIRECTORY_GUARDS} -- 3
|
|
|
|
A.2. Random values [Section:RANDOM]
|
|
|
|
Frequently, we want to randomize the expiration time of something
|
|
so that it's not easy for an observer to match it to its start
|
|
time. We do this by randomizing its start date a little, so that
|
|
we only need to remember a fixed expiration interval.
|
|
|
|
By RAND(now, INTERVAL) we mean a time between now and INTERVAL in
|
|
the past, chosen uniformly at random.
|
|
|
|
|
|
A.3. Why not a sliding scale of primaryness? [Section:CVP]
|
|
|
|
At one meeting, I floated the idea of having "primaryness" be a
|
|
continuous variable rather than a boolean.
|
|
|
|
I'm no longer sure this is a great idea, but I'll try to outline
|
|
how it might work.
|
|
|
|
To begin with: being "primary" gives it a few different traits:
|
|
|
|
1) We retry primary guards more frequently. [Section:RETRYING]
|
|
|
|
2) We don't even _try_ building circuits through
|
|
lower-priority guards until we're pretty sure that the
|
|
higher-priority primary guards are down. (With non-primary
|
|
guards, on the other hand, we launch exploratory circuits
|
|
which we plan not to use if higher-priority guards
|
|
succeed.) [Section:SELECTING]
|
|
|
|
3) We retry them all one more time if a circuit succeeds after
|
|
the net has been down for a while. [Section:ON_SUCCESS]
|
|
|
|
We could make each of the above traits continuous:
|
|
|
|
1) We could make the interval at which a guard is retried
|
|
depend continuously on its position in CONFIRMED_GUARDS.
|
|
|
|
2) We could change the number of guards we test in parallel
|
|
based on their position in CONFIRMED_GUARDS.
|
|
|
|
3) We could change the rule for how long the higher-priority
|
|
guards need to have been down before we call a
|
|
<usable_if_no_better_guard> circuit <complete> based on a
|
|
possible network-down condition. For example, we could
|
|
retry the first guard if we tried it more than 10 seconds
|
|
ago, the second if we tried it more than 20 seconds ago,
|
|
etc.
|
|
|
|
I am pretty sure, however, that if these are worth doing, they
|
|
need more analysis! Here's why:
|
|
|
|
* They all have the potential to leak more information about a
|
|
guard's exact position on the list. Is that safe? Is there
|
|
any way to exploit that? I don't think we know.
|
|
|
|
* They all seem like changes which it would be relatively
|
|
simple to make to the code after we implement the simpler
|
|
version of the algorithm described above.
|
|
|
|
A.3. Controller changes
|
|
|
|
We will add to control-spec.txt a new possible circuit state, GUARD_WAIT,
|
|
that can be given as part of circuit events and GETINFO responses about
|
|
circuits. A circuit is in the GUARD_WAIT state when it is fully built,
|
|
but we will not use it because a circuit with a better guard might
|
|
become built too.
|
|
|
|
A.4. Persistent state format
|
|
|
|
The persistent state format doesn't need to be part of this
|
|
specification, since different implementations can do it
|
|
differently. Nonetheless, here's the one Tor uses:
|
|
|
|
The "state" file contains one Guard entry for each sampled guard
|
|
in each instance of the guard state (see section 2). The value
|
|
of this Guard entry is a set of space-separated K=V entries,
|
|
where K contains any nonspace character except =, and V contains
|
|
any nonspace characters.
|
|
|
|
Implementations must retain any unrecognized K=V entries for a
|
|
sampled guard when they regenerate the state file.
|
|
|
|
The order of K=V entries is not allowed to matter.
|
|
|
|
Recognized fields (values of K) are:
|
|
|
|
"in" -- the name of the guard state instance that this
|
|
sampled guard is in. If a sampled guard is in two guard
|
|
states instances, it appears twice, with a different "in"
|
|
field each time. Required.
|
|
|
|
"rsa_id" -- the RSA id digest for this guard, encoded in
|
|
hex. Required.
|
|
|
|
"bridge_addr" -- If the guard is a bridge, its configured address and
|
|
port (this can be the ORPort or a pluggable transport port). Optional.
|
|
|
|
"nickname" -- the guard's nickname, if any. Optional.
|
|
|
|
"sampled_on" -- the date when the guard was sampled. Required.
|
|
|
|
"sampled_by" -- the Tor version that sampled this guard.
|
|
Optional.
|
|
|
|
"unlisted_since" -- the date since which the guard has been
|
|
unlisted. Optional.
|
|
|
|
"listed" -- 0 if the guard is not listed; 1 if it is. Required.
|
|
|
|
"confirmed_on" -- date when the guard was
|
|
confirmed. Optional.
|
|
|
|
"confirmed_idx" -- position of the guard in the confirmed
|
|
list. Optional.
|
|
|
|
"pb_use_attempts", "pb_use_successes", "pb_circ_attempts",
|
|
"pb_circ_successes", "pb_successful_circuits_closed",
|
|
"pb_collapsed_circuits", "pb_unusable_circuits",
|
|
"pb_timeouts" -- state for the circuit path bias algorithm,
|
|
given in decimal fractions. Optional.
|
|
|
|
All dates here are given as a (spaceless) ISO8601 combined date
|
|
and time in UTC (e.g., 2016-11-29T19:39:31).
|
|
|
|
|
|
TODO. Still non-addressed issues [Section:TODO]
|
|
|
|
Simulate to answer: Will this work in a dystopic world?
|
|
|
|
Simulate actual behavior.
|
|
|
|
For all lifetimes: instead of storing the "this began at" time,
|
|
store the "remove this at" time, slightly randomized.
|
|
|
|
Clarify that when you get a <complete> circuit, you might need to
|
|
relaunch circuits through that same guard immediately, if they
|
|
are circuits that have to be independent.
|
|
|
|
|
|
Fix all items marked XX or TODO.
|
|
|
|
"Directory guards" -- do they matter?
|
|
|
|
Suggestion: require that all guards support downloads via BEGINDIR.
|
|
We don't need to worry about directory guards for relays, since we
|
|
aren't trying to prevent relay enumeration.
|
|
|
|
IP version preferences via ClientPreferIPv6ORPort
|
|
|
|
Suggestion: Treat it as a preference when adding to
|
|
{CONFIRMED_GUARDS}, but not otherwise.
|
|
|