Winsock Multiple Connections

[emyztik]

I'm having trouble accepting multiple connections in Winsock.

Here's the code:

Private Sub Form_Load()
    wsServer(0).LocalPort = 4500
    wsServer(0).Listen
End Sub

Private Sub wsServer_Close(Index As Integer)
    wsServer(Index).Close
End Sub

Private Sub wsServer_ConnectionRequest(Index As Integer, ByVal requestID As Long)
    Dim K As Integer

    For K = 1 To wsServer.UBound
        If wsServer(K).State = sckClosed Then Exit For
    Next K

    If K = wsServer.UBound + 1 Then
        Load wsServer(wsServer.UBound + 1)
    End If

    wsServer(K).Accept requestID
End Sub

It accepts one(1) connection but a second(2nd) cannot connect.

Am I doing something wrong?

[DigiRev]

I usually do it a little differently but try this:

Private Sub wsServer_ConnectionRequest(Index As Integer, ByVal requestID As Long)
    Dim K As Integer
    
    If wsServer.ubound = 0 Then
        Load wsServer(1)
        wsServer(1).Accept requestID
    Else
        For K = 1 To wsServer.ubound
            If wsServer(K).State = sckClosed Then Exit For
        Next K
    
        If K = wsServer.ubound Then
            K = K + 1
            Load wsServer(K)
        End If
    
        wsServer(K).Accept requestID
    End If
End Sub

[emyztik]

Thanks for the reply.

It says: "Control array element '2' doesn't exist", on the code below:

wsServer(K).Accept requestID

[Doogle]

I think there was a typo in DigiRev's example. The snippet of code prior to the Load should look like this, I believe

        '
        ' If a closed socket wasn't found
        ' then K will be the element number we
        ' want to load. If we did find a closed socket
        ' then element K is already loaded
        '
        If K > wsServer.ubound Then
            Load wsServer(K)
        End If

[DigiRev]

Well, according to that code, k will never be > than .UBound

Try this

Private Function NextSocket() As Integer
    Dim i As Integer, intRet As Integer
    If wsServer.UBound = 0 Then
        Load wsServer(1)
        NextSocket = 1
    Else
        For i = 1 To wsServer.UBound
            If wsServer(i).State = sckClosed Then
                intRet = i
                Exit For
            End If
        Next i
        If intRet > 0 Then
            NextSocket = intRet
        Else
            intRet = wsServer.UBound + 1
            Load wsServer(intRet)
            NextSocket = intRet
        End If
    End If
End Function

Private Sub wsServer_ConnectionRequest(Index As Integer, ByVal requestID As Long)
    Dim i As Integer
    i = NextSocket
    If i > 0 Then
        wsServer(i).Accept requestID
    End If
End Sub

[Doogle]

Well, according to that code, k will never be > than .UBound

I beg to differ

Private Sub Form_Load()
Dim intI As Integer
Dim intK As Integer
intK = 100
For intI = 1 To 10
    If intI = intK Then Exit For
Next intI
Debug.Print intI
End Sub

If you run the above you'll see that intI = 11. When a For / Next loop completes, the loop control variable is one more then the ending condition. (for positive values). This is because the increment and test happen at the 'Next'.

[leinad31]

If wsServer.ubound = 0 Then is unnecessary since K iteration starts at one. My winsock is a bit hazy but wouldn't it be better (from debugging POV) to reset port to zero first so it will be auto-assigned (e.g. cases wherein listening port number assigned at design time rather than run-time so all created instances will default to that port number instead of zero)?

Private Sub wsServer_ConnectionRequest(Index As Integer, ByVal requestID As Long)
   Dim K As Integer
    
   For K = 1 To wsServer.ubound
      If wsServer(K).State = sckClosed Then Exit For
   Next K
    
   If K > wsServer.ubound Then
      Load wsServer(K)
   End If
   
   wsServer(K).LocalPort = 0    
   wsServer(K).Accept requestID
End Sub

[DigiRev]

I beg to differ

Private Sub Form_Load()
Dim intI As Integer
Dim intK As Integer
intK = 100
For intI = 1 To 10
    If intI = intK Then Exit For
Next intI
Debug.Print intI
End Sub

If you run the above you'll see that intI = 11. When a For / Next loop completes, the loop control variable is one more then the ending condition. (for positive values). This is because the increment and test happen at the 'Next'.

ah you're right

I thought it'd end at 10, my mistake

[DigiRev]

If wsServer.ubound = 0 Then is unnecessary since K iteration starts at one. My winsock is a bit hazy but wouldn't it be better (from debugging POV) to reset port to zero first so it will be auto-assigned (e.g. cases wherein listening port number assigned at design time rather than run-time so all created instances will default to that port number instead of zero)?

Private Sub wsServer_ConnectionRequest(Index As Integer, ByVal requestID As Long)
   Dim K As Integer
    
   For K = 1 To wsServer.ubound
      If wsServer(K).State = sckClosed Then Exit For
   Next K
    
   If K > wsServer.ubound Then
      Load wsServer(K)
   End If
   
   wsServer(K).LocalPort = 0    
   wsServer(K).Accept requestID
End Sub

All Winsocks in the array will use the same local port as the first one

[Doogle]

Generally it's not a good idea to set the LocalPort property in anything other than the Listening socket. TCP will allocate an almost random local port if none is specified. This is because it can take up to 4 minutes for a used local port to be released. Attempting to re-use the local port within 4 mins. can cause unexpected errors.

(see here: http://support.microsoft.com/kb/173619)

[leinad31]

Yes, your reusing socket with localport assigned (the for loop), so localport = 0 is necessary to prevent address in use.

[Doogle]

No, the Port is in use by the Server Program and all the sockets should (must)communicate via the same Local Port (that's the Port the client is connecting to). The TCP port is the unique TCP 'address' of the application. Thus something like '192.168.0.12:4000' (IP Address:Tcp Port) uniquely defines a TCP application on the Network.

Within the application the port will be connected to multiple clients. If you attempted to start another application using the same local port you'd get the 'Address in use' message. Basically, the IP Address identifies the Machine and the TCP Port identifies the Application. Each must be unique within a given Computer.

[leinad31]

K. I was thinking of local test using loopback 127.0.0.1. It would error then.

[Doogle]

No it wouldn't. Let's say 127.0.0.1 is listening on Port 4000 (that would be the 'Listner' applications LocalPort). The 'connecting application' would set it's RemotePort to 4000 - leaving the 'connecting' application's LocalPort to be allocated by TCP, eg 23456. Hence no conflict in LocalPort usage.

The listner's TCP Address is 127.0.0.1:4000 the connecting application is 127.0.0.1:23456. If another instance of the connecting application is run on the same machine, TCP will allocate a LocalPort that is not in use, eg 23457, and that application, as far as TCP is concerned, is unique. (127.0.0.1:23457) However, they're both communicating with the server's port 4000 through their RemotePort setting.

[reisve]

I can't recall exactely, but had a similar problem. There is a property you set in the WINSOCK so every new connection will get a new (ramdom?) port. Otherwise, Winsock will only accept a connection at the time.

[Doogle]

If you don't set the LocalPort property in the 'connecting' application, TCP will allocate a free one. That ensures that you will not confilict with any other open Local Ports. If you set the Local Port yourself then you run the risk of conflicts and "Address in Use" errors and the "can't resuse for 4 minutes" 'feature'.

[emyztik]

So, which should fix the problem? Now I'm confused

[leinad31]

Private Sub wsServer_ConnectionRequest(Index As Integer, ByVal requestID As Long)
   Dim K As Integer
    
   For K = 1 To wsServer.ubound
      If wsServer(K).State = sckClosed Then Exit For
   Next K
    
   If K > wsServer.ubound Then
      Load wsServer(K)
   End If
   
   wsServer(K).Accept requestID
End Sub

[dilettante]

For TCP the end doing a Listen is the "server" and must be sure that it sets LocalPort to the port to Listen on. This can be done explicitly (LocalPort property), via a Bind method parameter, or a Listen method parameter. The Listen operation does not use RemotePort.

The end doing the Connect is the "client" and should set LocalPort to 0 and RemotePort to the server's listening port. This can be done explicitly or via a parameter to the Connect method. When the Connect begins TCP sees the 0 and assigns an available ephemeral port number (if available) automatically.

When the server Accepts the connection request, the Accepting Winsock gets its LocalPort set to the Listen port (automatically) and its RemotePort set to the client's ephemeral port (automatically).


Every TCP "connection" is identified internally by TCP as a 4-tuple (2 endpoint pairs): [[LocalIP, LocalPort], [RemoteIP, RemotePort]]. If a client fails to reset its LocalPort (preferably to 0) before connecting to the same server endpoint [RemoteIP, RemotePort] again it will get an error. Even if the client sets LocalPort to 0, if the ephemeral port pool is exhausted it will get an error. Windows normally reserves ~ 4000 ports per IP address as its ephemeral port pool. Most PCs only have one IP address though.

This is what takes a "4 minute delay" to be retired: the entry in TCP's connection list. It is a TCP/IP standard and meant to help TCP "connections" survive brief interruptions in lower-level connectivity between the endpoints.

See TCP Connection States and Netstat Output or dig out your Comer books.

[DigiRev]

So, which should fix the problem? Now I'm confused

Use this:

Well, according to that code, k will never be > than .UBound

Try this

Private Function NextSocket() As Integer
    Dim i As Integer, intRet As Integer
    If wsServer.UBound = 0 Then
        Load wsServer(1)
        NextSocket = 1
    Else
        For i = 1 To wsServer.UBound
            If wsServer(i).State = sckClosed Then
                intRet = i
                Exit For
            End If
        Next i
        If intRet > 0 Then
            NextSocket = intRet
        Else
            intRet = wsServer.UBound + 1
            Load wsServer(intRet)
            NextSocket = intRet
        End If
    End If
End Function

Private Sub wsServer_ConnectionRequest(Index As Integer, ByVal requestID As Long)
    Dim i As Integer
    i = NextSocket
    If i > 0 Then
        wsServer(i).Accept requestID
    End If
End Sub