I have implemented NSURLSession
for downloading fairly large files from our servers. Now as long as I\'m working in foreground or background and go back to the app
I think that after your application did force-quit you should start all over again (.
If the user terminates your app, the system cancels any pending tasks.
and
When all of the tasks associated with a background session are complete, the system relaunches a terminated app (assuming that the sessionSendsLaunchEvents property was set to YES and that the user did not force quit the app)
https://developer.apple.com/library/ios/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html