bittorrent was the first client written for the protocol. before version 6.0, bittorrent was written in python, and was free software. the source code for versions up to and including 3.4.2 were distributed under the mit license and versions 4.x and 5.x were released under the bittorrent open source license, a modified version of the jabber open source license. since version 6.0, bittorrent is a rebranded version of µtorrent. as a result, its source is closed.