Question

Using C#, does broadcasting over UDP repeatedly send its packet, or just once?

I’ve never used this technology before and I want to temporarily broadcast small bit of info (a small one line string) over the LAN. If the receiving end isn’t ready will the broadcast repeat itself or was it a one time thing? The code I’m using is from here. So what I want to start the Broadcaster one one machine and a few minutes later start the receiver and retrieve what the broadcaster sent.

Here is the code

using System;  
using System.Net;  
using System.Net.Sockets;  
using System.Text;  
class Broadcst  
{  
  public static void Main()  
  {  
   Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Dgram,  
                ProtocolType.Udp);  
   IPEndPoint iep1 = new IPEndPoint(IPAddress.Broadcast, 9050);  
   IPEndPoint iep2 = new IPEndPoint(IPAddress.Parse("192.168.1.255"), 9050);  
   string hostname = Dns.GetHostName();  
   byte[] data = Encoding.ASCII.GetBytes(hostname);  
   sock.SetSocketOption(SocketOptionLevel.Socket,SocketOptionName.Broadcast, 1);  
   sock.SendTo(data, iep1);  
   sock.SendTo(data, iep2);  
   sock.Close();  
  }  
}  

Answer 1

UDP by design only sends a packet once. It has no concept of handshakes (unlike TCP), error correction, or transmission guarantees. You can’t even be sure that your data gets to where you send it unless you manually request checksums or something like that.

Wikipedia has a nice section on this: Reliability and congestion control solutions in UDP.

So, yes, you will need to implement transmission guarantee code if you want reliability. But what if the message from the recipient saying that the data was received is delayed? Well, then you need to implement some kind of timeout. What if the message gets lost? You need to resend the data to the recipient. How do you know if the recipient gets it this time? Etc…

If you don’t want to do this, then I’d suggest looking into TCP which automatically manages this for you.