There is a lightweight acknowledgement scheme, and a scheme for transferring directories by wrapping the file contents in simple headers. You'll notice that -f and -t (for 'from' and 'to') are not in the scp manpages. In SCP, once the client is connected to the server, and all the authentication and authorization has been done, the client sends the remote shell a command like scp -f myfile.txt, which just writes the contents of the file myfile.txt to the stream (for the client to read) or scp -t myfile.txt which reads from the stream and writes to myfile.txt. SCP is file transfer done using only that shell and a remote command. That's what you're using when you ssh to a server and type commands at the remote shell's prompt. The most familiar two processes would be a shell (at the server) and an interactive terminal emulator (at the client). The SSH protocol creates a secure tunnel through which you can transfer a bidirectional stream, and you can use that stream to connect any two processes you like.

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