Server-push-to-client application that uses port 80 only - .NET?


I'm looking for a way to write a .NET client/server application that uses port 80 to push communication from the server to the client.  An example would be a chat application with these characteristics:
 - runs on the client all the time.
 - the server decides when to send new information to a client
 - upon receiving this push from the server, the client reacts by popping up on the screen.

Some notes:
My guess is that I need to maintain an open session over port 80 - but I don't think it needs to be http.  The "client" can not run as a server, however - in other words, I can't install a web-server-lite on the client and expect it to respond to incoming http requests: corporate firewalls would prevent that.  Also, a client-pull will not work because clients have to react to new information on the server within 4 seconds, and that's too many requests for both the client and the server - both would get bogged down.  I've seen an application that seems to run completely on port 80 without client pull's called BoldChat ( and I would think that the way this program works would apply here as well.  

Any suggestions on how I would go about doing this with a .NET (or Java, or other) client?
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Here is where I found good examples for getting started in VB.Net
Look at the sample app. called VB.NET How-To Sockets Client
Just change the port num to 80


Link for ASP programs... 

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One other way, not that good one, but works. if you want to use webservices.
I once made a Webservice and called a Webmethod's say BeginXXX method. Now that is an asyncronous method, The control is returned to the client. The webservice holds the control on its side.. when ever some thing is to be returned to the client, server returns from that method XXX, and the controls EndXXX method is called. Which at the end initiates the BeginXXX again. So it keeps on moving in an Infinite loop

Hope it makes a little sence..
I am writing an article on it. If I somehow complete it. I will place it somewhere for everyone :)
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