iOS 整个网络请求系统分为这几部分：

  • URL Loading:
    • NSURLRequest / NSMutableURLRequest
    • NSURLResponse / NSHTTPURLResponse
    • NSURLConnection
  • Cache Management
    • NSURLCache
    • NSCachedURLResponse
  • Authentication and Credentials
    • NSURLCredential
    • NSURLCredentialStorage
    • NSURLAuthenticationChallenge
    • NSURLAuthenticationChallengeSender
    • NSURLProtectionSpace
  • Cookie Storage
    • NSHTTPCookie
    • NSHTTPCookieStorage
  • Protocol Support

参考 URL Loading System Overview.