I have an async server socket listening to some port. Then from another pc, I connect to the server and send 1 byte. Everything works fine, but there's a strange behavior. When I pull of network cable and try to send 1 byte (before os realizes cable was pulled of), I don't get any exception/error and as expected, server don't receive that packet. Is this how sockets are supposed to work? Does this mean that in case of connection loss some packets can be lost (because I don't get an exception and don't know that request was not sent)? Here's the code:
private void button3_Click(object sender, EventArgs e)
{
var b = new byte[1] {1};
client.BeginSend(b, 0, b.Length, 0, new AsyncCallback(SendCallback), client);
}
private void SendCallback(IAsyncResult ar)
{
Socket client = (Socket)ar.AsyncState;
int bytesSent = client.EndSend(ar);
this.Invoke(new MethodInvoker(() => { MessageBox.Show(bytesSent.ToString() + " bytes sent"); }));
}
Windows at socket layer maintains a socket buffer at kernel. A successful send at application layer simply means the data is copied into the kernel buffer. The data in this buffer is pushed by the TCP stack to the remote application. In windows, if a packet is dropped, TCP resends the data for 3 times after which the TCP stack notifies the connection close to the application. Interval between retries is decided by the RTT. First retry is after 1*RTT, second is after 2*RTT & third after 3*RTT. In your case, the 1 byte you send simply copied into the kernel buffer & indicated success. It will take 3*RTT to indicate the socket is closed. This connection close is notified only if you invoke any socket API or monitoring the socket for close event. After pulling the cable, if you queue a second send exactly after 3*RTT, send should through the exception. Another way to get indication of the send failure immediately is by setting the send socket buffer size to zero (SetSocketOption(..,SendBuffer,..)) so that the TCP stack directly use your buffer & indicates a failure immediately.