2017-cho-churn
findings extracted from this paper
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32 of 108 identified censoring ASes leak their censorship policies to other ASes, and 18 leak to other countries. Sweden's AS1299 leaked censorship to 9 countries including the United States, Ukraine, and Singapore; China's AS4812 leaked to 5 countries. Censorship leakage occurs when a transit AS implements filtering that affects traffic for users outside the censor's jurisdiction.
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Censors in Russia, Iran, and India implement all three measured censorship techniques simultaneously: block pages, RST injection, and TTL anomalies. Iran and Cyprus censoring ASes censor content across many URL categories (including General News, Internet Services, Pornography, Gambling), while most other censoring ASes restrict only a few category types.
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Network-level path churn is critical for censor localization: 25%, 30%, 38%, and 67% of ICLab source-destination pairs observe distinct AS-level path changes over periods of one day, week, month, and year respectively. Without path churn, nearly 90% of constructed CNFs return five or more solutions (ambiguous), compared to less than 2% when multiple distinct paths are included.
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Combining boolean network tomography with BGP path churn from the ICLab platform identifies 108 censoring ASes located in 49 countries across 4.9M measurements, reducing the candidate set of potential censoring ASes by 97% on average. 97.9% of constructed SAT CNFs return exactly one solution enabling exact AS-level censor identification, with less than 0.7% returning no solution.
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Splitting measurement data by individual URL and time granularity (day, week, month) is necessary for SAT solvability: coarser time granularity reduces solvability because censorship policies change and noise accumulates, producing unsolvable CNFs. The authors solved 34,298 CNFs in total, each averaging 43 clauses and 17.41ms to solve using an off-the-shelf SAT solver (picosat).