NSUInteger cacheSizeMemory = 5*1024*1024; // 5 MB
NSUInteger cacheSizeDisk = 5*1024*1024; // 5 MB
NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:cacheSizeMemory diskCapacity:cacheSizeDisk diskPath:@"nsurlcache"];
[NSURLCache setSharedURLCache:sharedCache];

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.example.apple-samplecode.SimpleBackgroundTransfer.BackgroundSession"];
configuration.URLCache = [NSURLCache sharedURLCache];
configuration.requestCachePolicy = NSURLRequestReturnCacheDataElseLoad;