The Internet has undergone dramatic changes in the past 15 years, and nowforms a global communication platform that billions of users rely on for theirdaily activities. While this transformation has brought tremendous benefits tosociety, it has also created new threats to online privacy, ranging fromprofiling of users for monetizing personal information to nearly omnipotentgovernmental surveillance. As a result, public interest in systems foranonymous communication has drastically increased. Several such systems havebeen proposed in the literature, each of which offers anonymity guarantees indifferent scenarios and under different assumptions, reflecting the pluralityof approaches for how messages can be anonymously routed to their destination.Understanding this space of competing approaches with their differentguarantees and assumptions is vital for users to understand the consequences ofdifferent design options. In this work, we survey previous research on designing, developing, anddeploying systems for anonymous communication. To this end, we provide ataxonomy for clustering all prevalently considered approaches (includingMixnets, DC-nets, onion routing, and DHT-based protocols) with respect to theirunique routing characteristics, deployability, and performance. This, inparticular, encompasses the topological structure of the underlying network;the routing information that has to be made available to the initiator of theconversation; the underlying communication model; and performance-relatedindicators such as latency and communication layer. Our taxonomy andcomparative assessment provide important insights about the differences betweenthe existing classes of anonymous communication protocols, and it also helps toclarify the relationship between the routing characteristics of theseprotocols, and their performance and scalability.
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