FINDING · EVALUATION

Never-once avoidance succeeds for 75% of source-destination pairs that do not already terminate in the US (a highly routing-central country) at δ=0.5, and for nearly all pairs avoiding less central countries. Russia is the hardest case at ~35% success (δ=0.5) due to proximity to the dense European node cluster. The median successful source-destination pair has over 1,000 valid DeTor circuits when avoiding the US and 500 when avoiding China.

From 2017-li-detor — DeTor: Provably Avoiding Geographic Regions in Tor · §6.2.1, §6.2.4 · 2017 · USENIX Security Symposium

Implications

Geographic avoidance of routing-central nations (US, China) is feasible in practice at the cost of a reduced but still large circuit pool; routing-adjacent nations (Russia) are harder to avoid and may require accepting lower proof coverage.
Circuit pre-filtering using the ratio of maximum-acceptable-distance to minimum-circuit-distance is a viable a priori predictor of proof success, reducing wasted connection setup attempts.