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Tor Path Specification
Roger Dingledine
Nick Mathewson
Note: This is an attempt to specify Tor as currently implemented. Future
versions of Tor will implement improved algorithms.
This document tries to cover how Tor chooses to build circuits and assign
streams to circuits. Other implementations MAY take other approaches, but
implementors should be aware of the anonymity and load-balancing implications
of their choices.
THIS SPEC ISN'T DONE YET.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
1. General operation
Tor begins building circuits as soon as it has enough directory
information to do so (see section 5 of dir-spec.txt). Some circuits are
built preemptively because we expect to need them later (for user
traffic), and some are built because of immediate need (for user traffic
that no current circuit can handle, for testing the network or our
reachability, and so on).
When a client application creates a new stream (by opening a SOCKS
connection or launching a resolve request), we attach it to an appropriate
open circuit if one exists, or wait if an appropriate circuit is
in-progress. We launch a new circuit only
if no current circuit can handle the request. We rotate circuits over
time to avoid some profiling attacks.
To build a circuit, we choose all the nodes we want to use, and then
construct the circuit. Sometimes, when we want a circuit that ends at a
given hop, and we have an appropriate unused circuit, we "cannibalize" the
existing circuit and extend it to the new terminus.
These processes are described in more detail below.
This document describes Tor's automatic path selection logic only; path
selection can be overridden by a controller (with the EXTENDCIRCUIT and
ATTACHSTREAM commands). Paths constructed through these means may
violate some constraints given below.
1.1. Terminology
A "path" is an ordered sequence of nodes, not yet built as a circuit.
A "clean" circuit is one that has not yet been used for any traffic.
A "fast" or "stable" or "valid" node is one that has the 'Fast' or
'Stable' or 'Valid' flag
set respectively, based on our current directory information. A "fast"
or "stable" circuit is one consisting only of "fast" or "stable" nodes.
In an "exit" circuit, the final node is chosen based on waiting stream
requests if any, and in any case it avoids nodes with exit policy of
"reject *:*". An "internal" circuit, on the other hand, is one where
the final node is chosen just like a middle node (ignoring its exit
policy).
A "request" is a client-side stream or DNS resolve that needs to be
served by a circuit.
A "pending" circuit is one that we have started to build, but which has
not yet completed.
A circuit or path "supports" a request if it is okay to use the
circuit/path to fulfill the request, according to the rules given below.
A circuit or path "might support" a request if some aspect of the request
is unknown (usually its target IP), but we believe the path probably
supports the request according to the rules given below.
1.1. A server's bandwidth
Old versions of Tor did not report bandwidths in network status
documents, so clients had to learn them from the routers' advertised
server descriptors.
For versions of Tor prior to 0.2.1.17-rc, everywhere below where we
refer to a server's "bandwidth", we mean its clipped advertised
bandwidth, computed by taking the smaller of the 'rate' and
'observed' arguments to the "bandwidth" element in the server's
descriptor. If a router's advertised bandwidth is greater than
MAX_BELIEVABLE_BANDWIDTH (currently 10 MB/s), we clipped to that
value.
For more recent versions of Tor, we take the bandwidth value declared
in the consensus, and fall back to the clipped advertised bandwidth
only if the consensus does not have bandwidths listed.
2. Building circuits
2.1. When we build
2.1.1. Clients build circuits preemptively
When running as a client, Tor tries to maintain at least a certain
number of clean circuits, so that new streams can be handled
quickly. To increase the likelihood of success, Tor tries to
predict what circuits will be useful by choosing from among nodes
that support the ports we have used in the recent past (by default
one hour). Specifically, on startup Tor tries to maintain one clean
fast exit circuit that allows connections to port 80, and at least
two fast clean stable internal circuits in case we get a resolve
request or hidden service request (at least three if we _run_ a
hidden service).
After that, Tor will adapt the circuits that it preemptively builds
based on the requests it sees from the user: it tries to have two fast
clean exit circuits available for every port seen within the past hour
(each circuit can be adequate for many predicted ports -- it doesn't
need two separate circuits for each port), and it tries to have the
above internal circuits available if we've seen resolves or hidden
service activity within the past hour. If there are 12 or more clean
circuits open, it doesn't open more even if it has more predictions.
Only stable circuits can "cover" a port that is listed in the
LongLivedPorts config option. Similarly, hidden service requests
to ports listed in LongLivedPorts make us create stable internal
circuits.
Note that if there are no requests from the user for an hour, Tor
will predict no use and build no preemptive circuits.
The Tor client SHOULD NOT store its list of predicted requests to a
persistent medium.
2.1.2. Clients build circuits on demand
Additionally, when a client request exists that no circuit (built or
pending) might support, we create a new circuit to support the request.
For exit connections, we pick an exit node that will handle the
most pending requests (choosing arbitrarily among ties), launch a
circuit to end there, and repeat until every unattached request
might be supported by a pending or built circuit. For internal
circuits, we pick an arbitrary acceptable path, repeating as needed.
In some cases we can reuse an already established circuit if it's
clean; see Section 2.3 (cannibalizing circuits) for details.
2.1.3. Servers build circuits for testing reachability and bandwidth
Tor servers test reachability of their ORPort once they have
successfully built a circuit (on start and whenever their IP address
changes). They build an ordinary fast internal circuit with themselves
as the last hop. As soon as any testing circuit succeeds, the Tor
server decides it's reachable and is willing to publish a descriptor.
We launch multiple testing circuits (one at a time), until we
have NUM_PARALLEL_TESTING_CIRC (4) such circuits open. Then we
do a "bandwidth test" by sending a certain number of relay drop
cells down each circuit: BandwidthRate * 10 / CELL_NETWORK_SIZE
total cells divided across the four circuits, but never more than
CIRCWINDOW_START (1000) cells total. This exercises both outgoing and
incoming bandwidth, and helps to jumpstart the observed bandwidth
(see dir-spec.txt).
Tor servers also test reachability of their DirPort once they have
established a circuit, but they use an ordinary exit circuit for
this purpose.
2.1.4. Hidden-service circuits
See section 4 below.
2.1.5. Rate limiting of failed circuits
If we fail to build a circuit N times in a X second period (see Section
2.3 for how this works), we stop building circuits until the X seconds
have elapsed.
XXXX
2.1.6. When to tear down circuits
XXXX
2.2. Path selection and constraints
We choose the path for each new circuit before we build it. We choose the
exit node first, followed by the other nodes in the circuit. All paths
we generate obey the following constraints:
- We do not choose the same router twice for the same path.
- We do not choose any router in the same family as another in the same
path.
- We do not choose more than one router in a given /16 subnet
(unless EnforceDistinctSubnets is 0).
- We don't choose any non-running or non-valid router unless we have
been configured to do so. By default, we are configured to allow
non-valid routers in "middle" and "rendezvous" positions.
- If we're using Guard nodes, the first node must be a Guard (see 5
below)
- XXXX Choosing the length
For "fast" circuits, we only choose nodes with the Fast flag. For
non-"fast" circuits, all nodes are eligible.
For all circuits, we weight node selection according to router bandwidth.
We also weight the bandwidth of Exit and Guard flagged nodes depending on
the fraction of total bandwidth that they make up and depending upon the
position they are being selected for.
These weights are published in the consensus, and are computed as described
in Section 3.4.3 of dir-spec.txt. They are:
Wgg - Weight for Guard-flagged nodes in the guard position
Wgm - Weight for non-flagged nodes in the guard Position
Wgd - Weight for Guard+Exit-flagged nodes in the guard Position
Wmg - Weight for Guard-flagged nodes in the middle Position
Wmm - Weight for non-flagged nodes in the middle Position
Wme - Weight for Exit-flagged nodes in the middle Position
Wmd - Weight for Guard+Exit flagged nodes in the middle Position
Weg - Weight for Guard flagged nodes in the exit Position
Wem - Weight for non-flagged nodes in the exit Position
Wee - Weight for Exit-flagged nodes in the exit Position
Wed - Weight for Guard+Exit-flagged nodes in the exit Position
Wgb - Weight for BEGIN_DIR-supporting Guard-flagged nodes
Wmb - Weight for BEGIN_DIR-supporting non-flagged nodes
Web - Weight for BEGIN_DIR-supporting Exit-flagged nodes
Wdb - Weight for BEGIN_DIR-supporting Guard+Exit-flagged nodes
Wbg - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
Wbm - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
Wbe - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
Wbd - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests
Additionally, we may be building circuits with one or more requests in
mind. Each kind of request puts certain constraints on paths:
- All service-side introduction circuits and all rendezvous paths
should be Stable.
- All connection requests for connections that we think will need to
stay open a long time require Stable circuits. Currently, Tor decides
this by examining the request's target port, and comparing it to a
list of "long-lived" ports. (Default: 21, 22, 706, 1863, 5050,
5190, 5222, 5223, 6667, 6697, 8300.)
- DNS resolves require an exit node whose exit policy is not equivalent
to "reject *:*".
- Reverse DNS resolves require a version of Tor with advertised eventdns
support (available in Tor 0.1.2.1-alpha-dev and later).
- All connection requests require an exit node whose exit policy
supports their target address and port (if known), or which "might
support it" (if the address isn't known). See 2.2.1.
- Rules for Fast? XXXXX
2.2.1. Choosing an exit
If we know what IP address we want to connect to or resolve, we can
trivially tell whether a given router will support it by simulating
its declared exit policy.
Because we often connect to addresses of the form hostname:port, we do not
always know the target IP address when we select an exit node. In these
cases, we need to pick an exit node that "might support" connections to a
given address port with an unknown address. An exit node "might support"
such a connection if any clause that accepts any connections to that port
precedes all clauses (if any) that reject all connections to that port.
Unless requested to do so by the user, we never choose an exit server
flagged as "BadExit" by more than half of the authorities who advertise
themselves as listing bad exits.
2.2.2. User configuration
Users can alter the default behavior for path selection with configuration
options.
- If "ExitNodes" is provided, then every request requires an exit node on
the ExitNodes list. (If a request is supported by no nodes on that list,
and StrictExitNodes is false, then Tor treats that request as if
ExitNodes were not provided.)
- "EntryNodes" and "StrictEntryNodes" behave analogously.
- If a user tries to connect to or resolve a hostname of the form
<target>.<servername>.exit, the request is rewritten to a request for
<target>, and the request is only supported by the exit whose nickname
or fingerprint is <servername>.
2.3. Cannibalizing circuits
If we need a circuit and have a clean one already established, in
some cases we can adapt the clean circuit for our new
purpose. Specifically,
For hidden service interactions, we can "cannibalize" a clean internal
circuit if one is available, so we don't need to build those circuits
from scratch on demand.
We can also cannibalize clean circuits when the client asks to exit
at a given node -- either via the ".exit" notation or because the
destination is running at the same location as an exit node.
2.4. Learning when to give up ("timeout") on circuit construction
Since version 0.2.2.8-alpha, Tor attempts to learn when to give up on
circuits based on network conditions.
2.4.1 Distribution choice and parameter estimation
Based on studies of build times, we found that the distribution of
circuit build times appears to be a Frechet distribution. However,
estimators and quantile functions of the Frechet distribution are
difficult to work with and slow to converge. So instead, since we
are only interested in the accuracy of the tail, we approximate
the tail of the distribution with a Pareto curve.
We calculate the parameters for a Pareto distribution fitting the data
using the estimators in equation 4 from:
http://portal.acm.org/citation.cfm?id=1647962.1648139
This is:
alpha_m = s/(ln(U(X)/Xm^n))
where s is the total number of completed circuits we have seen, and
U(X) = x_max^u * Prod_s{x_i}
with x_i as our i-th completed circuit time, x_max as the longest
completed circuit build time we have yet observed, u as the
number of unobserved timeouts that have no exact value recorded,
and n as u+s, the total number of circuits that either timeout or
complete.
Using log laws, we compute this as the sum of logs to avoid
overflow and ln(1.0+epsilon) precision issues:
alpha_m = s/(u*ln(x_max) + Sum_s{ln(x_i)} - n*ln(Xm))
This estimator is closely related to the parameters present in:
http://en.wikipedia.org/wiki/Pareto_distribution#Parameter_estimation
except they are adjusted to handle the fact that our samples are
right-censored at the timeout cutoff.
Additionally, because this is not a true Pareto distribution, we alter
how Xm is computed. The Xm parameter is computed as the midpoint of the most
frequently occurring 50ms histogram bin, until the point where 1000
circuits are recorded. After this point, the weighted average of the top
'cbtnummodes' (default: 3) midpoint modes is used as Xm. All times below
this value are counted as having the midpoint value of this weighted average bin.
The timeout itself is calculated by using the Pareto Quantile function (the
inverted CDF) to give us the value on the CDF such that 80% of the mass
of the distribution is below the timeout value.
Thus, we expect that the Tor client will accept the fastest 80% of
the total number of paths on the network.
2.4.2. How much data to record
From our observations, the minimum number of circuit build times for a
reasonable fit appears to be on the order of 100. However, to keep a
good fit over the long term, we store 1000 most recent circuit build times
in a circular array.
The Tor client should build test circuits at a rate of one per
minute up until 100 circuits are built. This allows a fresh Tor to have
a CircuitBuildTimeout estimated within 1.5 hours after install,
upgrade, or network change (see below).
Timeouts are stored on disk in a histogram of 50ms bin width, the same
width used to calculate the Xm value above. This histogram must be shuffled
after being read from disk, to preserve a proper expiration of old values
after restart.
2.4.3. How to record timeouts
Circuits that pass the timeout threshold should be allowed to continue
building until a time corresponding to the point 'cbtclosequantile'
(default 95) on the Pareto curve, or 60 seconds, whichever is greater.
The actual completion times for these circuits should be recorded.
Implementations should completely abandon a circuit and record a value
as an 'unknown' timeout if the total build time exceeds this threshold.
The reason for this is that right-censored pareto estimators begin to lose
their accuracy if more than approximately 5% of the values are censored.
Since we wish to set the cutoff at 20%, we must allow circuits to continue
building past this cutoff point up to the 95th percentile.
2.4.4. Detecting Changing Network Conditions
We attempt to detect both network connectivity loss and drastic
changes in the timeout characteristics.
We assume that we've had network connectivity loss if 3 circuits
timeout and we've received no cells or TLS handshakes since those
circuits began. We then temporarily set the timeout to 60 seconds
and stop counting timeouts.
If 3 more circuits timeout and the network still has not been
live within this new 60 second timeout window, we then discard
the previous timeouts during this period from our history.
To detect changing network conditions, we keep a history of
the timeout or non-timeout status of the past 20 circuits that
successfully completed at least one hop. If more than 90% of
these circuits timeout, we discard all buildtimes history, reset
the timeout to 60, and then begin recomputing the timeout.
If the timeout was already 60 or higher, we double the timeout.
2.4.5. Consensus parameters governing behavior
Clients that implement circuit build timeout learning should obey the
following consensus parameters that govern behavior, in order to allow
us to handle bugs or other emergent behaviors due to client circuit
construction. If these parameters are not present in the consensus,
the listed default values should be used instead.
cbtdisabled
Default: 0
Min: 0
Max: 1
Effect: If 1, all CircuitBuildTime learning code should be
disabled and history should be discarded. For use in
emergency situations only.
cbtnummodes
Default: 3
Min: 1
Max: 20
Effect: This value governs how many modes to use in the weighted
average calculation of Pareto parameter Xm. A value of 3 introduces
some bias (2-5% of CDF) under ideal conditions, but allows for better
performance in the event that a client chooses guard nodes of radically
different performance characteristics.
cbtrecentcount
Default: 20
Min: 3
Max: 1000
Effect: This is the number of circuit build times to keep track of
for the following option.
cbtmaxtimeouts
Default: 18
Min: 3
Max: 10000
Effect: When this many timeouts happen in the last 'cbtrecentcount'
circuit attempts, the client should discard all of its
history and begin learning a fresh timeout value.
cbtmincircs
Default: 100
Min: 1
Max: 10000
Effect: This is the minimum number of circuits to build before
computing a timeout.
cbtquantile
Default: 80
Min: 10
Max: 99
Effect: This is the position on the quantile curve to use to set the
timeout value. It is a percent (10-99).
cbtclosequantile
Default: 95
Min: Value of cbtquantile parameter
Max: 99
Effect: This is the position on the quantile curve to use to set the
timeout value to use to actually close circuits. It is a percent
(0-99).
cbttestfreq
Default: 60
Min: 1
Max: 2147483647 (INT32_MAX)
Effect: Describes how often in seconds to build a test circuit to
gather timeout values. Only applies if less than 'cbtmincircs'
have been recorded.
cbtmintimeout
Default: 2000
Min: 500
Max: 2147483647 (INT32_MAX)
Effect: This is the minimum allowed timeout value in milliseconds.
The minimum is to prevent rounding to 0 (we only check once
per second).
cbtinitialtimeout
Default: 60000
Min: Value of cbtmintimeout
Max: 2147483647 (INT32_MAX)
Effect: This is the timeout value to use before computing a timeout,
in milliseconds.
2.5. Handling failure
If an attempt to extend a circuit fails (either because the first create
failed or a subsequent extend failed) then the circuit is torn down and is
no longer pending. (XXXX really?) Requests that might have been
supported by the pending circuit thus become unsupported, and a new
circuit needs to be constructed.
If a stream "begin" attempt fails with an EXITPOLICY error, we
decide that the exit node's exit policy is not correctly advertised,
so we treat the exit node as if it were a non-exit until we retrieve
a fresh descriptor for it.
XXXX
3. Attaching streams to circuits
When a circuit that might support a request is built, Tor tries to attach
the request's stream to the circuit and sends a BEGIN, BEGIN_DIR,
or RESOLVE relay
cell as appropriate. If the request completes unsuccessfully, Tor
considers the reason given in the CLOSE relay cell. [XXX yes, and?]
After a request has remained unattached for SocksTimeout (2 minutes
by default), Tor abandons the attempt and signals an error to the
client as appropriate (e.g., by closing the SOCKS connection).
XXX Timeouts and when Tor auto-retries.
* What stream-end-reasons are appropriate for retrying.
If no reply to BEGIN/RESOLVE, then the stream will timeout and fail.
4. Hidden-service related circuits
XXX Tracking expected hidden service use (client-side and hidserv-side)
5. Guard nodes
We use Guard nodes (also called "helper nodes" in the literature) to
prevent certain profiling attacks. Here's the risk: if we choose entry and
exit nodes at random, and an attacker controls C out of N servers
(ignoring bandwidth), then the
attacker will control the entry and exit node of any given circuit with
probability (C/N)^2. But as we make many different circuits over time,
then the probability that the attacker will see a sample of about (C/N)^2
of our traffic goes to 1. Since statistical sampling works, the attacker
can be sure of learning a profile of our behavior.
If, on the other hand, we picked an entry node and held it fixed, we would
have probability C/N of choosing a bad entry and being profiled, and
probability (N-C)/N of choosing a good entry and not being profiled.
When guard nodes are enabled, Tor maintains an ordered list of entry nodes
as our chosen guards, and stores this list persistently to disk. If a Guard
node becomes unusable, rather than replacing it, Tor adds new guards to the
end of the list. When choosing the first hop of a circuit, Tor
chooses at
random from among the first NumEntryGuards (default 3) usable guards on the
list. If there are not at least 2 usable guards on the list, Tor adds
routers until there are, or until there are no more usable routers to add.
A guard is unusable if any of the following hold:
- it is not marked as a Guard by the networkstatuses,
- it is not marked Valid (and the user hasn't set AllowInvalid entry)
- it is not marked Running
- Tor couldn't reach it the last time it tried to connect
A guard is unusable for a particular circuit if any of the rules for path
selection in 2.2 are not met. In particular, if the circuit is "fast"
and the guard is not Fast, or if the circuit is "stable" and the guard is
not Stable, or if the guard has already been chosen as the exit node in
that circuit, Tor can't use it as a guard node for that circuit.
If the guard is excluded because of its status in the networkstatuses for
over 30 days, Tor removes it from the list entirely, preserving order.
If Tor fails to connect to an otherwise usable guard, it retries
periodically: every hour for six hours, every 4 hours for 3 days, every
18 hours for a week, and every 36 hours thereafter. Additionally, Tor
retries unreachable guards the first time it adds a new guard to the list,
since it is possible that the old guards were only marked as unreachable
because the network was unreachable or down.
Tor does not add a guard persistently to the list until the first time we
have connected to it successfully.
6. Router descriptor purposes
There are currently three "purposes" supported for router descriptors:
general, controller, and bridge. Most descriptors are of type general
-- these are the ones listed in the consensus, and the ones fetched
and used in normal cases.
Controller-purpose descriptors are those delivered by the controller
and labelled as such: they will be kept around (and expire like
normal descriptors), and they can be used by the controller in its
CIRCUITEXTEND commands. Otherwise they are ignored by Tor when it
chooses paths.
Bridge-purpose descriptors are for routers that are used as bridges. See
doc/design-paper/blocking.pdf for more design explanation, or proposal
125 for specific details. Currently bridge descriptors are used in place
of normal entry guards, for Tor clients that have UseBridges enabled.
X. Old notes
X.1. Do we actually do this?
How to deal with network down.
- While all helpers are down/unreachable and there are no established
or on-the-way testing circuits, launch a testing circuit. (Do this
periodically in the same way we try to establish normal circuits
when things are working normally.)
(Testing circuits are a special type of circuit, that streams won't
attach to by accident.)
- When a testing circuit succeeds, mark all helpers up and hold
the testing circuit open.
- If a connection to a helper succeeds, close all testing circuits.
Else mark that helper down and try another.
- If the last helper is marked down and we already have a testing
circuit established, then add the first hop of that testing circuit
to the end of our helper node list, close that testing circuit,
and go back to square one. (Actually, rather than closing the
testing circuit, can we get away with converting it to a normal
circuit and beginning to use it immediately?)
[Do we actually do any of the above? If so, let's spec it. If not, let's
remove it. -NM]
X.2. A thing we could do to deal with reachability.
And as a bonus, it leads to an answer to Nick's attack ("If I pick
my helper nodes all on 18.0.0.0:*, then I move, you'll know where I
bootstrapped") -- the answer is to pick your original three helper nodes
without regard for reachability. Then the above algorithm will add some
more that are reachable for you, and if you move somewhere, it's more
likely (though not certain) that some of the originals will become useful.
Is that smart or just complex?
X.3. Some stuff that worries me about entry guards. 2006 Jun, Nickm.
It is unlikely for two users to have the same set of entry guards.
Observing a user is sufficient to learn its entry guards. So, as we move
around, entry guards make us linkable. If we want to change guards when
our location (IP? subnet?) changes, we have two bad options. We could
- Drop the old guards. But if we go back to our old location,
we'll not use our old guards. For a laptop that sometimes gets used
from work and sometimes from home, this is pretty fatal.
- Remember the old guards as associated with the old location, and use
them again if we ever go back to the old location. This would be
nasty, since it would force us to record where we've been.
[Do we do any of this now? If not, this should move into 099-misc or
098-todo. -NM]
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