  Internet censors seek ways to identify and block internet access to
information they deem objectionable. Increasingly, censors deploy advanced
networking tools such as deep-packet inspection (DPI) to identify such
connections. In response, activists and academic researchers have developed and
deployed network traffic obfuscation mechanisms. These apply specialized
cryptographic tools to attempt to hide from DPI the true nature and content of
connections.
  In this survey, we give an overview of network traffic obfuscation and its
role in circumventing Internet censorship. We provide historical and technical
background that motivates the need for obfuscation tools, and give an overview
of approaches to obfuscation used by state of the art tools. We discuss the
latest research on how censors might detect these efforts. We also describe the
current challenges to censorship circumvention research and identify concrete
ways for the community to address these challenges.
