Description
By default a viewer/client use TCP port 5900 to connect to a server (or 5800 for browser access). Alternatively, a server can connect to a viewer in "listening mode" on port 5500. One advantage of listening mode is that the server site does not have to configure its firewall/NAT to allow access on port 5900 (or 5800); the burden is on the viewer, which is useful if the server site has no computer expertise, while the viewer user would be expected to be more knowledgeable.
Although RFB started as a relatively simple protocol it has been enhanced with additional features (such as file transfers) and more sophisticated compression and security techniques as it has developed. To maintain seamless cross-compatibility between the many different VNC client and server implementations, the clients and servers negotiate a connection using the best RFB version, and the most appropriate compression and security options, that they can both support.
Read more about this topic: RFB Protocol
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