Plugins are executable programs on the client platform.
The SSH client must be manually configured to use a plugin for a particular connection. The configuration takes the form of a command line, including the location of the plugin executable, and optionally command-line arguments that are meaningful to the particular plugin.
The client invokes the plugin as a subprocess, passing it a pair of 8-bit-clean pipes as its standard input and output. On those pipes, the client and plugin will communicate via the protocol specified below.