Handling connection attempts in asynchronous sockets

[tempmail1]

Hi everybody,
while working with my first Socket-relateg project I came across with the following problem.
I have two apps: client and server. The Server receives commands from a third-part app and pushes commands to selected client app. Client app should process received command and send back a reply. I want my server app to work asynchronously, so that after receiving a command (accepting new connection and adding it to a client list) it should immediately start waiting for another incomming connections.
Everything looks fine untill the number of incoming connections reaches about 20 per second (or rather the period of time between connection calls is short enough). In this case the third-paty app keeps getting the following error:
"Only one usage of each socket address (protocol/network address/port) is normally permitted <IP_Address>:<port>"

I wanted to use the Socket.Poll method but all of my attempts have failed. I don't really know how to make the server to wait untill listener socket is free and can accept incoming connections.



The following is part of my Server source code:

private bool StartListening()
{
try
{
serverSocket = new Socket(AddressFamily.InterNetwork,
SocketType.Stream,
ProtocolType.Tcp);

IPEndPoint ipEndPoint = new IPEndPoint(IPAddress.Any, this.portServer);

serverSocket.Bind(ipEndPoint);
serverSocket.Listen(4);
serverSocket.BeginAccept(new AsyncCallback(OnConnectRequest), serverSocket);
}
catch (Exception ex)
{
[...]
return false;
}
return true;
}


public void OnConnectRequest(IAsyncResult ar)
{
Socket listener = (Socket)ar.AsyncState;
Socket socketCommunication = null;
lock (listener)
{

bool bNewClientCreated = true;
try
{
socketCommunication = listener.EndAccept(ar);
// NewConnection - adds clientSocket to list of clients,
// sets up OnDataReceived callback which receives and processes
// received command
NewConnection(socketCommunication);
}
catch (SocketException sockExc)
{
bNewClientCreated = false;
[...]
}
catch (Exception exc)
{
bNewClientCreated = false;
[...]
}

if (bNewClientCreated)
{
listener.BeginAccept(new AsyncCallback(OnConnectRequest), listener);
}
else
{
// closes client socket and removes it from the list
CloseClientSocket(socketCommunication);
}
}
}

I will appreciate any help as I ran out of ideas.
Regards
Rob

[MikeAThon]

The fault might caused by the 3rd party application's machine (i.e., your server code might be fine). The application's machine might be using up all its ephemeral ports, and not recycling them quickly enough. Ephemeral ports are the port numbers assigned by the OS to client applications when they make outgoing connections. The range of the port numbers can theoretically be as high as 1025-65000, but the default setting is only 1025-5000. In addition, once a port number has been used, it cannot be reused (for a connection to the same server) for at least the TIME_WAIT period, which defaults to 4 minutes.

So, with these default values, the 3rd party app might be exhausting the ephemeral port range and not recycling them quickly enough.

On the machine that's running the 3rd party application, try editing the registry to adjust (or add) the MaxUserPort entry and the TcpTimedWaitDelay entry. For a description of these entries, and a KB article that describes a similar problem, see "Description of TCP/IP settings that you may have to adjust when SQL Server connection pooling is disabled" at http://support.microsoft.com/kb/328476/

Mike