Popup from main page

I have a web site application written in VB.net and asp. The main page uses JavaScript to load a popup page that performs multiple tasks. The main page accepts a variety of incoming messages, only SOME of which need to be passed to this popup page for processing. I found an ugly way to pass the message to the popup by placing it in a Session variable. Now I need to cause it to be handled. I see two possibilities, and I do not have a clue how to implement either. Either one would be great:

1. In the main page, trigger a subroutine in the popup, using JavaScript.
OR
2. In the popup, establish a trigger any time that Session variable changes.

Ummm...you can't change the value of the session variable from JavaScript. You would have to send a message back to ASP.NET asking *it* to change the value.

Question: You say the popup page performs multiple tasks. Does it do so in JavaScript, in the browser, or are they performed back on the server, in ASP.NET code?

And the main page: You say it accepts a variety of messages. Again, are those messaes received by JavaScript code or ASP.NET code?

As far as I can see these messages comes from ASP.Net.

Not important where they come from. Important what code *receives* them.

And there were two questions there. Need both answers.