The RTIP-32 FTP Client defaults to passive mode for best firewall compatibility. The client finally connects, using the client-side data port, to the unprivileged data port (contained in the PASV command) on the server side. The server responds to the client using a PASV command containing the IP address and the port number that the server is listening on for the client to connect. The client sends an empty PASV command telling the server that it wants to run in passive mode. After connecting, the client begins listening on a data port determined by adding one to the clientВ’s command port. This is done by having the client initially connect to the server on the command port 21 using an arbitrary unprivileged port. It enables a data connection to be made from the client on an unprivileged port to an unprivileged port on the server side. An FTP client in passive mode is used as a work-around. Many firewalls will not permit a server to connect out on a privileged port, such as 20, and are thus not compatible with active mode.

The server connects back to the client using its data port 20 to the clientВ’s port, which was in the PORT command. The client then begins to listen on a port determined by adding one to this client command port and then sends a PORT command containing this information to the server over the command port. The client will first establish a connection to the server's command port 21 from an arbitrary unprivileged port. In active mode, the FTP client will restrain the connection to using the standard server ports of 20 and 21. File I/O is implemented through a user-supplied callback function, allowing applications to use the FTP clients even if they do not use a real file system. The client is provided through a small set of API calls. It also provides directory listing and working directory management routines. FTP, the File Transport Protocol, is a client/server protocol that allows a client to transfer files to and from a server.

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