Shared instance of NSHTTPCookieStorage does not persist cookies

I'm developing an application where the server hands me a cookie to identify the user. My successive requests need to have that cookie to have the response that the user expects. What I can't understand is how and when the shared instance of NSHTTPCookieStorage loses its cookies. The first solution I used is to archive and save the cookies from my server to user defaults on application termination

[iRSS]

HTTPCookieStorage.shared.cookies はシミュレーターでは動かない 実機で試したらとれた

[Utmrer]

NSUserDefaultsにCookie保存するのやめろ、って言ってる。