UDP Client receives only 1 message

Question

I have a server client application that I am currently working on. The server is receiving data fine over a WAN and the client seems to receive the data, but the client is only receiving one communication. Is there anything over a WAN that would make a client always only receive the first return UDP communication and none of the subsequent. Thanks for the help.

Client UDP Listening code

private void receiveUDP()
    {
        System.Net.IPEndPoint test = new System.Net.IPEndPoint(System.Net.IPAddress.Any,UDP_PORT_NUMBER);
        System.Net.EndPoint serverIP = (System.Net.EndPoint)test;
        server.Bind(serverIP);
        //server.Ttl = 50;

        EndPoint RemoteServ = (EndPoint)listenUDP;
        do
        {
            byte[] content = new byte[1024];
            int data = server.ReceiveFrom(content, ref RemoteServ);

            string message = Encoding.ASCII.GetString(content);


            ProcessCommands(message);


        } while (true);
    }

Answer 1

This is a bit of a stab in the dark (since you don't provide enough code to really say what's going on definitively), but there's one major reason why you might consistently see some UDP datagrams not be delivered over a WAN, while others always arrive successfully. This reason is MTU; the Maximum Transmission Unit which can be sent in a single UDP datagram. This can easily produce behaviour such as what you're seeing if (for example), your first datagram is a short "I accept your connection" message, and you then follow that with a datagrams containing large files; the first (small) datagram is smaller than the MTU and is delivered, while the following (large) datagrams are larger than the MTU, and are discarded en route.

For UDP over a WAN, the MTU will not be higher than about 1500 bytes, and in many situations may be as low as 1200 bytes. Any packets larger than that will be silently dropped somewhere between endpoints. To send large blocks of data via UDP, you need to chop them up into pieces smaller than the MTU for the network segment across which you're transmitting them.

On a LAN, you can usually get away with sending datagrams of any size. But as soon as they're being sent over the Internet or otherwise through heterogenous networks, they're likely to be silently discarded.

If you do need to send large files, you might choose to transmit them via TCP instead; TCP automatically manages chopping data up to fit within the MTU, and ensures that its packets are all received, and are received in order; guarantees that you will not receive from datagrams sent via UDP.

As I mentioned above, this is a complete stab in the dark and may not actually be related to your actual troubles. But it's the elephant in the room, when all we have to go on is that the first packet always arrives successfully, and later packets never do.