This duality justifies the separation of FTP into control and data channels and its reply code structure. The commands in Table 2 are presented in the typical order of use. FTP sessions are stateful, modifying a single context for current authenticated user, pathname, and transfer parameters. Authentication, in particular, was generalized to multiple rounds of challenges and responses by RFC 2228. Choreographing this ballet are the reply codes.
Duality reveals itself again in replies.
The actual transfers in turn migrated to data connections formatted for automata alone. That requires applications to carefully synchronize access to the transfer channel more care than queuing request and reply messages that package together the complete context of each transaction. The File Transfer Protocol Control and data channels still fused together. Introduced Create cmd. July 1973 File Transfer Protocol Separate control and data connections.