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+<a id="path-spec.txt-1"></a>
+
+# General operation
+
+Tor begins building circuits as soon as it has
+[enough directory information](./when-we-build.md) to do so.
+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).
+
+```text
+  [Newer versions of Tor (0.2.6.2-alpha and later):
+   If the consensus contains Exits (the typical case), Tor will build both
+   exit and internal circuits. When bootstrap completes, Tor will be ready
+   to handle an application requesting an exit circuit to services like the
+   World Wide Web.
+```
+
+If the consensus does not contain Exits, Tor will only build internal
+circuits. In this case, earlier statuses will have included "internal"
+as indicated above. When bootstrap completes, Tor will be ready to handle
+an application requesting an internal circuit to hidden services at
+".onion" addresses.
+
+If a future consensus contains Exits, exit circuits may become available.\]
+
+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.
+
+<a id="path-spec.txt-1.1"></a>
+
+## 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.
+
+<a id="path-spec.txt-1.2"></a>
+
+## A relay's bandwidth {#bandwidth}
+
+Old versions of Tor did not report bandwidths in network status
+documents, so clients had to learn them from the routers' advertised
+relay descriptors.
+
+For versions of Tor prior to 0.2.1.17-rc, everywhere below where we
+refer to a relay'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 relay'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.