How Server will know when some client Get disconnected

Hi...

I am devloping a chat application.... there are loats of users. and one user can have many friends in his/her list. and showing in two groups as Online And Offline.

How Chat Server will know what is the user's Status(online or offline) When the user will get disconnected due to Internet failure

" Let you are a user having 10 friends, and when you loged in, at that time you have 5 online friends and 5 offline friends. lets after some time one of your online friend get disconnected from server due to any kind of netwrk failuer, then how server will know about it."

[604 byte] By [NIGAMa] at [2007-10-3 4:31:50]