Hi frds I need some Information.

I want to know the logic of working of any messenger which shows you the messeage as pop up whenever you receive a new message.

Example : - If you are login to Gtalk and you receieve a new mail then it gets pop up at right corner showing that you receive a new mail.
Now what I want to know is how server comes to know that which message is for whom and how it identifies whether that user is online and if online then how its correctly been pop up at that user machine that he has receive a new message.

We know try to do so to continuously send request to server whether there is any message for logedin user if so then send that message to user machine, but this consume more resources.
We are looking at routine at server only that if any message is for a user then server itself should pop up at that user machine just as what normally messenger do.
Also irrespective the user uses direct internet connection or use proxy but from server it should send the appropriate message to user for whom the message has came on server.
What details of receiver user should be needed on server to send a message to respective user.


Plzz frds hope you can understand what I am trying to explain and plzz give nay suggestion on this.
???