Sample code to resume downloading files on iOS?

I have an iOS application in which the user has the ability to download videos. I would like to be able to support the resumption of downloads when the download is interrupted by the network, by the user, or by any other circumstances. I understand that this will require changes both on the client side and on the server side. I was wondering if someone could point me in the right direction with sample code and / or documentation that I can read for tips on how to support this functionality? Something with hints in the correct separation, figuring out which piece was last sent after the interrupted connection, etc.