Tcp connection Keep alive

Question

i am creating a client server application. the server is already design and in place waiting for connection from the client. Now in the client section i would like to keep the connection alive throughout th life of the application and the connection only closes when the main client application close's or shutdown or the server closes it.

Currently every 10 seconds Server closes the TCP connection.I tried with

socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.KeepAlive, true);

but it doesn't work for me.. Below is my code block

public TCPStreamDevice(string RemoteIPAddress, int RemotePort, string SourceIPAddress, int SourcePortNo)        
{
    mIpAddress = RemoteIPAddress;
    mPort = RemotePort;

    mClient = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
    System.Net.IPEndPoint LocalEndPoint = new System.Net.IPEndPoint(System.Net.IPAddress.Parse(SourceIPAddress), SourcePortNo);

    mClient.Bind(LocalEndPoint);

    mDataReceivedCallback = new AsyncCallback(DataReceivedTCPCallback_Handler);
    mBuffer = new byte[1024];
    Description = new DeviceDescription();
}

and in the handler I have:

private void DataReceivedTCPCallback_Handler(IAsyncResult ar)
{
    try
    {
        Socket client = (Socket)ar.AsyncState;
        int bytesReceived = client.EndReceive(ar);

        if (bytesReceived > 0)
        {
            //to know transport level errors
            //EngineInterface.reponseReceived(mBuffer, false);

            ReceiveCallBackFunc(mBuffer, bytesReceived);

            client.BeginReceive(mBuffer, 0, 1024, SocketFlags.None, DataReceivedTCPCallback_Handler, client);
        }
        else
        {
            //disconnect
            /* when there is no datapacket  means no TCP connection is alive now (how can i keep Tcp alive here) */
        }
    }
}

Answer 1

In the call to SetSocketOption() , KeepAlive is not valid at the SocketOptionLevel.Tcp level, instead use SocketOptionLevel.Socket .

SetSocketOption( SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true ) ;

Answer 2

The comments and answer above are valid - sounds like a bad design choice to have a socket opened for the entire lifetime of the app AND expect things to work properly - you should build some sort of failsafe mechanism in case the connection gets dropped.

Back to keep-alives: You need them on both ends - server and client so check how the sockets are set up on both sides. I think that the default value for keep alives is 2 hours - that's a long time to wait for a keep-alive packet but it can be changed. Check Socket.IOControl method and use IOControlCode.KeepAliveValues with a structure that looks like this (unmanaged) http://msdn.microsoft.com/en-us/library/ms741621.aspx . More about control codes here: http://msdn.microsoft.com/en-us/library/system.net.sockets.iocontrolcode.aspx

Answer 3

The comment ("whrn there is no datapacket means no TCP connection") in your code is placed where you receive a disconnect (0 bytes) packet from the other side. There is no way to keep that connection alive because the other side choses to close it.

If the connection is being closed due to network issues, you would either get an exception, or it would seem as if the connection is valid but quiet.

Keep-alive mechanisms always work alongside with timeouts - the timeout enforces "if no data was received for x seconds, close the connection" where the keep-alive simply sends a dummy data packet to keep the timeout from occurring.

By implementing a protocol yourself (you're operating on the TCP/IP level) you only need to implement a keep-alive if you already have a timeout implemented on the other side.

Answer 4

You can set TCP keepAlives by setting the IOControlCode.KeepAliveValues when creating the socket on the server. The article below on code-project explains it very well.

Sample on How-To set keep alive values