NSURLSession background transfer timeout not fired

Question

When I try to start a basic NSURLSession transfer while the network is offline (air plane mode), with NSURLSessionConfiguration defaultSessionConfiguration and ephemeralSessionConfiguration, I of course immediately receive the NSError :
Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline."
But with NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier, the NSURLSession never returns, no response in any way, no timeout fired, it get stucked.
This happens with iOS 8.x. With iOS 7.x, I got the NSError as expected.
Why that ? Is there a way to get some error back ?

Answer 1

In general, an NSURLSession background session does not fail a task if something goes wrong on the wire. Rather, it continues looking for a good time to run the request and retries at that time. This continues until the resource timeout expires (that is, the value of the timeoutIntervalForResource property in the NSURLSessionConfiguration object you use to create the session). The current default for that value is one week! In other words, the behaviour of failing for a timeout in iOS7 was incorrect. In the context of a background session, it is more interesting to not fail immediately because of network problems. So since iOS8, NSURLSession task continues even if it encounters timeouts and network loss. It continues however until timeoutIntervalForResource is reached.

So basically timeoutIntervalForRequest won't work in Background session but timeoutIntervalForResource will.

I got this answer from one of the members of Apple Staff at the developer forum. Also, I have verified this by implementing.