Anonymous VoIP calls over the Internet holds great significance for privacy-conscious users, whistle-blowers and political activists alike. Prior research deems popular anonymization systems like Tor unsuitable for providing requisite performance guarantees that real-time applications like VoIP need. Their claims are backed by studies that may no longer be valid due to constant advancements in Tor. Moreover, we believe that these studies lacked the requisite diversity and comprehensiveness. Thus, conclusions from these studies led them to propose novel and tailored solutions. However, no such system is available for immediate use. Additionally, operating such new systems would incur significant costs for recruiting users and volunteered relays, to provide the necessary anonymity guarantees. It thus becomes imperative that the exact performance of VoIP over Tor be quantified and analyzed so that the potential performance bottlenecks can be amended. We thus conducted an extensive empirical study across various in-lab and real-world scenarios to shed light on VoIP performance over Tor. In over 0.5 million measurements spanning 12 months, across seven countries and covering about 6650 Tor relays, we observed that Tor supports good voice quality (Perceptual Evaluation of Speech Quality (PESQ) >3 and oneway delay <400ms) in more than 85% of cases. Further analysis indicates that in general for most Tor relays, the contentions due to cross-traffic were low enough to support VoIP calls, that are anyways transmitted at low rates (<120 Kbps). Our findings are supported by concordant measurements using iperf that show more than the adequate available bandwidth for most cases. Data published by the Tor Metrics also corroborates the same. Hence, unlike prior efforts, our research reveals that Tor is suitable for supporting anonymous VoIP calls.

中文翻译：

---

未走的路：重新思考通过 Tor 进行语音呼叫的可行性

互联网上的匿名 VoIP 呼叫对于注重隐私的用户、举报人和政治活动家等都具有重要意义。先前的研究认为流行的匿名系统（如 Tor）不适合提供 VoIP 等实时应用程序所需的必要性能保证。他们的主张得到了研究的支持，由于 Tor 的不断进步，这些研究可能不再有效。此外，我们认为这些研究缺乏必要的多样性和全面性。因此，这些研究的结论使他们提出了新颖且量身定制的解决方案。然而，没有这样的系统可以立即使用。此外，运行此类新系统将产生大量成本，用于招募用户和自愿中继，以提供必要的匿名保证。因此，必须量化和分析基于 Tor 的 VoIP 的确切性能，以便修正潜在的性能瓶颈。因此，我们对各种实验室内和实际场景进行了广泛的实证研究，以阐明 Tor 上的 VoIP 性能。在跨越 12 个月、跨越 7 个国家、覆盖约 6650 个 Tor 中继的超过 50 万次测量中，我们观察到 Tor 支持良好的语音质量（语音质量感知评估 (PESQ) >3 和单向延迟 <400ms），超过 85%案件。进一步的分析表明，对于大多数 Tor 中继而言，一般而言，由于交叉流量引起的争用足够低，足以支持 VoIP 呼叫，这些呼叫无论如何都以低速率 (<120 Kbps) 传输。我们的发现得到了使用 iperf 的一致测量的支持，在大多数情况下，这些测量显示出比足够的可用带宽更多。Tor Metrics 发布的数据也证实了这一点。因此，与之前的努力不同，我们的研究表明 Tor 适合支持匿名 VoIP 呼叫。