"The world of censorship is changing all the time.
It's a good idea to learn how to use bridges and pluggable transports before you actually need them."

"Just last week, ISPs in Belarus began blocking public Tor relays—but bridges and pluggable transports are so far working to defeat the blocks.
We are tracking other censorship events, such as those in Saudi Arabia, Kazakhstan, and elsewhere.
If you know details of these or any other Tor blocks, please tell us."
https://blog.torproject.org/tor-heart-bridges-and-pluggable-transports



   OBFS4 (The obfourscator)
https://github.com/Yawning/obfs4/blob/master/doc/obfs4-spec.txt  

This is a protocol obfuscation layer for TCP protocols.  
Its purpose is to keep a third party from telling what protocol is in use based on message contents.

   obfs4 offers protection against passive Deep Packet Inspection machines
   that expect the obfs4 protocol.  Such machines should not be able to verify
   the existence of the obfs4 protocol without obtaining the server's Node ID
   and identity public key.

   obfs4 offers protection against active attackers attempting to probe for
   obfs4 servers.  Such machines should not be able to verify the existence
   of an obfs4 server without obtaining the server's Node ID and identity
   public key.

   obfs4 offers protection against active attackers that have obtained the
   server's Node ID and identity public key.  Such machines should not be
   able to impersonate the server without obtaining the server's identity
   private key.

   obfs4 offers protection against some non-content protocol fingerprints,
   specifically the packet size, and optionally packet timing.

   obfs4 provides integrity and confidentiality of the underlying traffic,
   and authentication of the server.