NSURLSession在空闲时是否使用任何网络流量？

If I use an NSURLSession to download some files and once they are complete I just leave it sitting there idle (rather than invalidating it), does it consume any network traffic keeping a connection to the server open? In other words should I invalidate it and create a new one next time I need to download or just keep leave it idle waiting to be used again?If I use an NSURLSession to download some files