How does Report Viewer communicate with SSRS?

I'm working on an extranet solution that will contain some SSRS reports. We are leaning towards using report viewer to display the reports to the end user.

What I can't find out is exactly how the communic开发者_如何学运维ation is done. The client web browser will display the report, but where will it get the data from? Is it the client browser that directly accesses the SSRS web service, or is it the web server that hosts the report viewer that fetches the data from SSRS?

What this boils down to is whether my SSRS server needs to be visible on the Internet or if it is enough if the web server can reach the SSRS server.

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The SSRS connection is made from the webserver, nothing from the client.
The client makes postbacks to the web server on each update, a tip is to put the ReportViewer in an UpdatePanel to make the refresh using AJAX and not reloading the whole page.