There are two approaches for remotely controlling a computer, calling out and dialing in.
RealVNC and Microsoft's Remote Desktop are examples of the dialing in approach. When the server side of RealVNC is installed on a computer, it waits for an incoming connection. If the computer is behind a firewall, a hole needs to be punched in the firewall to allow this incoming connection.
People not wanting to deal with firewalls and port forwarding, can chose software, such as GoToMyPC that, like ET, phones home. This takes advantage of the fact that firewalls, as a rule, let anything out.
When the server side of GoToMyPC software is installed on a computer, it phones home to Citrix and maintains that connection at all times. A GoToMyPC customer, wanting to remotely control a computer, contacts Citrix, the company behind GoToMyPC. Citrix serves as a man in the middle and makes the connection between the two computers.
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