如何从互联网下载数据与简历选项…?

i am downloading content from server to my app. i am using NSUrlConnection to that in response i am getting data and i am storing that. But if user quits the app while downloading when he launches app next time i have to resume that download from where it stop(download remaining part). how to support this ... is any idea on how to handle this ....i am downloading content from server to my app.