About 20 years ago, with macOS 10.5 (Leopard), Apple introduced screen sharing. Apple did not invent a new protocol for screen sharing. Instead, it used the established VNC protocol. VNC is a pretty simple, unencrypted protocol using TCP port 5900. Historically, the protocol used a simple global password for authentication. Apple adapted the protocol for its own use, but overall, left the VNC protocol itself alone.
http://news.poseidon-us.com/TV3DQs
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