C＃我怎么知道是否发送了UDP数据包？

Question

i'm using this code

      bool NotSent = true;
        while (NotSent)
        {
            try
            {
                UdpClient udpServer = new UdpClient(port);
                udpServer.Connect(IPAddress.Parse("192.168.1.66"), port);
                Byte[] sendBytes = Encoding.ASCII.GetBytes("Hello");
                int res = udpServer.Send(sendBytes, sendBytes.Length);
                MessageBox.Show("Sent : " + res);
                udpServer.Close();
                NotSent = false;
            }
            catch (Exception ex) { MessageBox.Show("Error : " + ex.ToString()); continue; }
        }

so how i can know if "Hello" sent and received or not because all results always return 17

Answer 1

UDP does not implement an acknowledgement segment like TCP or other protocols.

UdpClient.Send() sends datagrams to the specified endpoint and returns the number of bytes successfully sent.

Thus, the 17 that you are seeing in res is telling you that 17 bytes have successfully been sent.

Source: https://msdn.microsoft.com/en-us/library/82dxxas0(v=vs.110).aspx

Answer 2

UDP is a connectionless protocol so there is no gurantee that the data was sent to its receipient.

One simple way to confirm that the data was indeed sent is to have the remote host send back an ack (acknowledgement) to the server.

Here is a simple implementation you can find from MSDN

// Sends a message to a different host using optional hostname and port parameters.
     UdpClient udpClientB = new UdpClient();
     udpClientB.Send(sendBytes, sendBytes.Length, "AlternateHostMachineName", 11000);

     //IPEndPoint object will allow us to read datagrams sent from any source.
     IPEndPoint RemoteIpEndPoint = new IPEndPoint(IPAddress.Any, 0);

     // Blocks until a message returns on this socket from a remote host.
     Byte[] receiveBytes = udpClient.Receive(ref RemoteIpEndPoint); 
     string returnData = Encoding.ASCII.GetString(receiveBytes);