I'm wondering how do all messenger programs (msn, google, yahoo, ...) notify me when a friend in my contact list sign in?
do they connect to a server periodically usgin a timer.
or does the server itself send a notification (data packet to my computer) ? what if my computer is behind a router? how this data reach me without forwarding the ports?
i do really need to know the answer since i've searched a lot without any helpful result.
thank u.