Why doesn't a zero-length Send() fail on a bad TCP socket connection?

Question

I'm trying to write a very simple TCP server in .NET, that deals with just one connection for one client, and my challenge is detecting a half-open connection.

As per this ubiquitous MSDN code...

// This is how you can determine whether a socket is still connected.
bool blockingState = client.Blocking;
try
{
    byte [] tmp = new byte[1];

    client.Blocking = false;
    client.Send(tmp, 0, 0);
    Console.WriteLine("Connected!");
}
catch (SocketException e) 
{
    // 10035 == WSAEWOULDBLOCK
    if (e.NativeErrorCode.Equals(10035))
        Console.WriteLine("Still Connected, but the Send would block");
    else
    {
        Console.WriteLine("Disconnected: error code {0}!", e.NativeErrorCode);
    }
}
finally
{
    client.Blocking = blockingState;
}

... I thought the intent of this was to uncover any real-world connection issue (such as the remote client's network cable being unplugged) and throw the desired exception.

Yet, I also understand that issuing a Send() simply causes the data to be put into the socket's underlying send buffer... so obviously it would not fail simply because of a problem on the client's side.

I've read that you need to do a follow-up Receive() to actually get the results of the zero-length send test. However, my server is currently using a continual BeginReceive approach, where, as soon as I actually receive data and process it, I start a BeginSend() and then issue another BeginReceive().

In other words, at the time that the "MSDN test" is performed, I already have a pending receive in effect. Wouldn't that somehow sense the lack of ACK and signal the error?

Thanks

Answer 1

Software wise the way to prove a connection is still alive, is to send a message to the other end and then wait until timeout for a response. It's not just that the connection is set up, but that the other end is responding usually.

Not really any different to the way a physical connection would be proven send 5v down one pair, see if about 5v comes back on the other.

Google TCP/IP Heartbeat for more help.