How to establish 2-way communication between a web server and a site server?

Question

I am planning a SaaS system, to be written in C#, ASP.NET using WCF that has two separate components:

1. On a static IP web server in the cloud will be a web app,

Answer 1

IIRC "push communication" is done by letting the client do a HTTP Request with an indefinate timeout. Then the server responds when he has something to say. After the respons the client immediately makes a new request.

It works out the same way like the server is making the connection and takes far less resources than polling.