commit a282bfc4996b39658b207e9ce1da322de26185cf Author: Nick Mathewson <nickm@torproject.org> Date: Fri Aug 27 11:11:08 2021 -0400 path-spec: describe the behavior of circuit_expire_building() This is my attempt to extract a general concise rule here. (I am not sure that Tor adjusts both timeout_ms and close_ms, but Arti does indeed adjust both.) --- path-spec.txt | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) diff --git a/path-spec.txt b/path-spec.txt index 0243260..34e3bec 100644 --- a/path-spec.txt +++ b/path-spec.txt @@ -481,7 +481,8 @@ of their choices. F(q) = Xm/((1.0-q)^(1.0/alpha)) - Thus, clients obtain their circuit build timeout by computing: + Thus, clients obtain the circuit build timeout for 3-hop circuits by + computing: timeout_ms = F(0.8) # 'cbtquantile' == 0.8 @@ -500,6 +501,27 @@ of their choices. timeout_ms = MIN(timeout_ms, max_observed_timeout) close_ms = MAX(MIN(close_ms, 2*max_observed_timeout), 'cbtinitialtimeout') +2.4.3. Calculating timeouts thresholds for circuits of different lengths + + The timeout_ms and close_ms estimates above are good only for 3-hop + circuits, since only 3-hop circuits are recorded in the list of build + times. + + To calculate the appropriate timeouts and close timeouts for circuits of + other lengths, the client multiples the timeout_ms and close_ms values + by a scaling factor determined by the number of communication hops + needed to build their circuits: + + timeout_ms[hops=n] = timeout_ms * Actions(N) / Actions(3) + + close_ms[hops=n] = close_ms * Actions(N) / Actions(3) + + where Actions(N) = N * (N + 1) / 2. + + To calculate timeouts for operations other than circuit building, + the client should add X to Actions(N) for every round-trip communication + required with the Xth hop. + 2.4.4. How to record timeouts Pareto estimators begin to lose their accuracy if the tail is omitted.