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+Filename: 151-path-selection-improvements.txt
+Title: Improving Tor Path Selection
+Version:
+Last-Modified:
+Author: Fallon Chen, Mike Perry
+Created: 5-Jul-2008
+Status: Draft
+
+Overview
+
+  The performance of paths selected can be improved by adjusting the
+  CircuitBuildTimeout and the number of guards. This proposal describes
+  a method of tracking buildtime statistics, and using those statistics
+  to adjust the CircuitBuildTimeout and the number of guards.
+
+Motivation
+
+  Tor's performance can be improved by excluding those circuits that
+  have long buildtimes (and by extension, high latency). For those Tor
+  users who require better performance and have lower requirements for
+  anonymity, this would be a very useful option to have.
+
+Implementation
+
+  Learning the CircuitBuildTimeout
+
+    Based on studies of build times, we found that the distribution of
+    circuit buildtimes appears to be a Pareto distribution. The number
+    of circuits to observe (ncircuits_to_observe) before changing the
+    CircuitBuildTimeout will be tunable. From our preliminary
+    measurements, it is likely that ncircuits_to_observe will be
+    somewhere on the order of 1000. The values can be represented
+    compactly in Tor in milliseconds as a circular array of 16 bit
+    integers. More compact long-term storage representations can be
+    implemented by simply storing a histogram with 50 millisecond
+    buckets when writing out the statistics to disk.
+
+  Calculating the preferred CircuitBuildTimeout
+
+    Circuits that have longer buildtimes than some x% of the estimated
+    CDF of the Pareto distribution will be excluded. x will be tunable
+    as well.
+
+  Circuit timeouts
+
+    In the event of a timeout, backoff values should include the 100-x%
+    of expected CDF of timeouts.  Also, in the event of network failure,
+    the observation mechanism should stop collecting timeout data.
+
+  Other notes
+
+    Since this follows a Pareto distribution, large reductions on the
+    timeout can be achieved without cutting off a great number of the
+    total paths.  However, hard statistics on which cutoff percentage
+    gives optimal performance have not yet been gathered.
+
+Issues
+
+  Impact on anonymity