path: root/doc/spec/path-spec.txt

                           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.

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 circuits that do not need to be "fast", when choosing among
   multiple candidates for a path element, we choose randomly.

   For "fast" circuits, we pick a given router as an exit with probability
   proportional to its bandwidth.

   For non-exit positions on "fast" circuits, we pick routers as above, but
   we weight the bandwidth of Exit-flagged nodes depending
   on the fraction of bandwidth available from non-Exit nodes.  Call the
   total bandwidth for Exit nodes under consideration E,
   and the total bandwidth for all nodes under
   consideration T.  If E<T/3, we do not consider Exit-flagged nodes.
   Otherwise, we weight their bandwidth with the factor (E-T/3)/E. This 
   ensures that bandwidth is evenly distributed over nodes in 3-hop paths.

   Similarly, guard nodes are weighted by the factor (G-T/3)/G, and not
   considered for non-guard positions if this value is less than 0.

   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. 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]