## 6 Limitations and Future Work  
In order to assess usability implications on a global scale, we treated Tor explicitly as a \"black box\" in this paper. We measured what end users around the world are confronted with in terms of performance. Our experiments do not aim for a detailed white-box analysis of Tor or technical improvements for latency, but serve as basis for assessing Tor usability based on tolerated waiting time. Important related literature on the technical details of improving Tor is presented in the Section 2. Moreover, the global experiments we conducted have some limitations in terms of node reliability, comparability, and estimation of page latency. When using the PlanetLab environment, traffic generated by other experiments on the same node could influence the experiment results. During the execution of the test scripts, the resources of the nodes have been shared with other experiments. Accordingly, the overall traffic speed might not be accurate and the performance of the direct and Tor connections  
Figure 13: Latency and Cancelation Rate for Anonymous Web Browsing (Survey Statements). should only be compared against each other and are (quantitatively) not exactly comparable across machines. However, the results from experiments conducted on the same node (_i.e._, direct _vs._ Tor) are still comparable since both traffic types used the same connection and the ratio between them was considered for analysis. On the one hand, our experiments might be overestimating the speed of normal web browsing, e.g., at home, because the PlanetLab environment provides in most cases server-grade computers with a good Internet connection. On the other hand, they may be underestimating the speed of normal web browsing because of the heavy load these PlanetLab nodes suffer. Though we suppose that these differences between PlanetLab nodes and common personal computers are insignificant, further research should include tests to strengthen this hypothesis. All in all, the relative ratio of both traffic types will be comparable. Our approach for calculating the extrapolation factor for downloading complete web pages, though most suited for our experimental setting, has some limitations: (i) The results vary between different websites, while extrapolating does not cover this issue. We do not consider this as crucial due to the fact that we focus on page latency; (ii) When downloading the complete web page, additional variations in terms of time and coverage for different browsers and individual browser settings may be experienced. An alternative method, which could better reflect a real user's browsing behavior, would be to provide a Tor exit node and use the requested websites for live measurement experiments. However, even though such alternative experiments could in theory be conducted without affecting the privacy of Tor users, this could nonetheless raise strong privacy concerns and potentially also cause legal issues in our university environment. We focused on clear-cut technical metrics that can be measured via automated requests. In the real world, the perceived latency of the user depends on various other aspects. Additional studies about influence factors for perceived latency such as cultural issues, the task at hand, or individual user settings of the browser or operating system could provide valuable information about how latency is experienced by users and what countermeasures could be applied, such as introducing a loading progress bar for Tor users. In future work, we plan a set of user studies on capturing those further, more individual or subjective aspects of latency acceptance and usability. In addition, we will investigate user acceptances correlation to educational and ethnical background and if IT knowledge has an influence on users acceptance. Furthermore, we will ask if risk awareness and risk aversion have an influence on the willingness to use Tor. First results in this direction were recently published by us [27].