The RFB protocol is used to transmit screen pixel data from one computer to another over a network, and send control events in return. It’s a simple yet powerful protocol invented by RealVNC.
VNC Server captures the desktop of the computer in real-time and sends it to VNC Viewer for display. VNC Viewer gathers your input (mouse, keyboard, or touch) and sends it for VNC Server to inject and actually achieve remote control.
You need a VNC Server for the remote computer you want to control, and a VNC Viewer for the computer or mobile device you want to control from. You can install and license these applications in advance, or download and run them on-demand, in the way that suits you best. Or do both, in different circumstances.
VNC Connect uses use the latest commercial version of the RFB protocol, originally defined as RFC 6143.
The current commercial version of the RFB protocol as used in VNC Connect adds a significant number of security, performance and functionality improvements over the protocol described in RFC6143.
For a technical explanation of the security of the RFB protocol used in VNC Connect, see https://www.realvnc.com/en/connect/vnc-connect-security-whitepaper/