An increasing number of censoring countries are using Deep Packet Inspection (DPI) to classify Internet traffic flows by protocol. While Tor uses bridge relays to get around a censor that blocks by IP address, the censor can use DPI to recognize and filter Tor traffic flows even when they connect to unexpected IP addresses.

Pluggable transports transform the Tor traffic flow between the client and the bridge. This way, censors who monitor traffic between the client and the bridge will see innocent-looking transformed traffic instead of the actual Tor traffic. External programs can talk to Tor clients and Tor bridges using the pluggable transport API, to make it easier to build interoperable programs.

Our goal is to have a wide variety of pluggable transport designs. Many are at the research phase now, so it's a perfect time to play with them or suggest new designs. Please let us know if you find or start other projects that could be useful for making Tor's traffic flows more DPI-resistant!