Question

I was told that UDP was connectionless meaning that you could not be sure if the packages would reach the destination.

Why when doing:

var dataToSend = new byte[]{1};
UdpClient client = new UdpClient();
client.Send(dataToSend,1,"192.168.0.45", 1234);

the variable LocalEndpoint initializes:

Correct me if I am wrong. I believe that the variable LocalEndPoint was initialized by the router. The reason why I believe that is because every time the server (192.168.0.45) receives data and then replies, I see that data is being send through the port 62446 on the reply.

So my question is if I am using the udp protocol why am I getting a response from the router? If I am getting a response from the router then that is not UDP or perhaps I have a wrong understanding of udp. I dont think the port number get’s randomly picked. If I have had configure the router to do port forwarding on port 62446 to some other computer then my program will not have had worked.

---

here is the client code:

string ipOfServer = "192.168.0.45";
int portServerIsListeningOn = 1234;

// send data to server
Socket sending_socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
IPEndPoint sending_end_point = new IPEndPoint(IPAddress.Parse(ipOfServer), portServerIsListeningOn);
sending_socket.SendTo(Encoding.ASCII.GetBytes("Test"), sending_end_point);

// after I send data localendpoint gets initialized! and the server always respond through that port!

// get info
var port = sending_socket.LocalEndPoint.ToString().Split(':')[1];

// now wait for server to send data back
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, int.Parse(port));
byte[] buffer = new byte[1024];
sending_socket.Receive(buffer); // <----- keeps waiting in here :(

here is the server code:

// wait for client to send data
UdpClient listener = new UdpClient(11000);
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, 11000);        
byte[] receive_byte_array = listener.Receive(ref groupEP);
listener.Connect(groupEP);

// reply
byte[] dataToSend = new byte[] { 1, 2, 3, 4, 5 };
listener.Send(dataToSend, dataToSend.Length);

Answer 1

The Bind is giving you the information contained in the LocalEndPoint property not the router:

This excerpt is from MSDN:

The LocalEndPoint property gets an EndPoint that contains the local IP
address and port number to which your Socket is bound. You must cast
this EndPoint to an IPEndPoint before retrieving any information. You
can then call the IPEndPoint.Address method to retrieve the local
IPAddress, and the IPEndPoint.Port method to retrieve the local port
number.

The LocalEndPoint property is usually set after you make a call to the
Bind method. If you allow the system to assign your socket’s local IP
address and port number, the LocalEndPoint property will be set after
the first I/O operation. For connection-oriented protocols, the first
I/O operation would be a call to the Connect or Accept method. For
connectionless protocols, the first I/O operation would be any of the
send or receive calls.

But you are correct in your understanding, UDP is a fire and forget style of sending data.