A co-worker is struggling with handling online/offline transitions in some networking code in a Xamarin (i.e. Mono, i.e. .NET) app for iOS. The code maintains a TCP socket to a server. If the device goes offline or into Airplane Mode, the code needs to close the socket, so that the higher levels of the app know it’s not online anymore.
The behavior he’s reporting is that, when the device is put into Airplane Mode, the socket acts as though it’s still connected. It doesn’t close, and (obviously) doesn’t receive any data, but most strangely the socket doesn’t fail after data is written to it. My knowledge of TCP tells me that if you write to a socket, but no ACK packet is received within ~15 seconds, the socket will be closed with an error (I think ECONNRESET?) But he swears this isn’t happening — it’s like the writes go to /dev/null. I’ve never seen this, but I’ve mostly been using NSURLSession or at least NSStream, not POSIX socket APIs, so the behavior may be different. My first guess was that Mono is doing something sketchy; he looked into the source code and found that it uses send() instead of write() to write to the TCP socket; is that kosher? The workaround he’s adopting is to run a SCNetworkReachability notifier, and to close the socket and go into offline mode when notified that the host is unreachable. I’m uncomfortable with this, since I know Reachability can be unreliable and this usage doesn’t go along with the best practices, i.e. using it just as a trigger to connect, not disconnect. So what’s the deal with Airplane Mode? I would assume that it takes down all network interfaces, which would close all sockets. But it doesn’t. Even so, I would assume that writing to a TCP socket would cause an error pretty soon; but apparently it doesn’t. Any advice? —Jens _______________________________________________ Do not post admin requests to the list. They will be ignored. Macnetworkprog mailing list (Macnetworkprog@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/macnetworkprog/archive%40mail-archive.com This email sent to arch...@mail-archive.com
