Question

I have a simple TCP server that communicates with some devices via GPRS. It works correctly without any problem for a long time. Nowdays there is a change in the devices (client devices, that send data) and they send lot more data than earlier and TCP message size can be 10 times bigger than earlier. Earlier it was max 1024 bytes/data message, now it can be more than 10 thousand bytes/message.

As I see – as the nature of TCP predicted – there are a lot of messages that are ‘broken’, so instead of a complete message only a part of that arrives, and later can come the other part etc.

I think my server – that is based on asynchronous pattern – handles this situation in a wrong way and I think the bug is in my RecieveCallBack function. I could not find to correct way to handle this situation. How can I do this?

Here is my RecieveCallBack function:

public void RecieveCallBack(IAsyncResult ar)
    {
        StateObject state = (StateObject)ar.AsyncState;

        try
        {
            Socket handler = state.WorkSocket;

            int read = handler.EndReceive(ar);

            if (read > 0)
            {
                var data = new byte[read];
                Array.Copy(state.Buffer, 0, data, 0, read);
            }
            else
            {
                if ((handler.Connected == false) || (handler.Available == 0))
                {
                    Close(state);
                    return;
                }
            }
        }
        catch (System.Net.Sockets.SocketException)
        {
            Close(state);
        }
        catch (System.Exception exc)
        {
            Debug.Assert(false, exc.Message);
            HandleSocketError(state, exc);
        }
    }

.net4/c#/vs2010

Thanks

UPDATE:
Clients are connected as they can be connected, so it is up to the GSM network. They can be connected even for days and they send data at every minutes. The protocol of the client devices is different, there are different kind of devices with different kind of protocols. One of them has delimiter and CRC at the end of the message, others does not have.

Answer 1

You need message framing. The protocol must specify how large the messages are – usually either a constant well-known size, a length prefix, or using message delimiters.