  The Internet has undergone dramatic changes in the past 15 years, and now
forms a global communication platform that billions of users rely on for their
daily activities. While this transformation has brought tremendous benefits to
society, it has also created new threats to online privacy, ranging from
profiling of users for monetizing personal information to nearly omnipotent
governmental surveillance. As a result, public interest in systems for
anonymous communication has drastically increased. Several such systems have
been proposed in the literature, each of which offers anonymity guarantees in
different scenarios and under different assumptions, reflecting the plurality
of approaches for how messages can be anonymously routed to their destination.
Understanding this space of competing approaches with their different
guarantees and assumptions is vital for users to understand the consequences of
different design options.
  In this work, we survey previous research on designing, developing, and
deploying systems for anonymous communication. To this end, we provide a
taxonomy for clustering all prevalently considered approaches (including
Mixnets, DC-nets, onion routing, and DHT-based protocols) with respect to their
unique routing characteristics, deployability, and performance. This, in
particular, encompasses the topological structure of the underlying network;
the routing information that has to be made available to the initiator of the
conversation; the underlying communication model; and performance-related
indicators such as latency and communication layer. Our taxonomy and
comparative assessment provide important insights about the differences between
the existing classes of anonymous communication protocols, and it also helps to
clarify the relationship between the routing characteristics of these
protocols, and their performance and scalability.
