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Demystifying UDP

Yesterday I was taking a look at [UDP Multicast], and attempting to try it out in C#. Unfortunately, I got a little bit confused as to how it worked, and ended up sending a couple of hours wondering what I did wrong. I'm writing this post to hopefully save you the trouble of fiddling around trying to get it to work yourself.

UDP stands for User Datagram Protocol (or Unreliable Datagram Protocol). It offers no guarantee that message sent will be received at the other end, but is usually faster than its counterpart, TCP. Each UDP message has a source and a destination address, a source port, and a destination port.

When you send a message to a multicast address (like the 239.0.0.0/8 range or the FF00::/8 range for ipv6, but that's a little bit more complicated), your router will send a copy of the message to all the other interested hosts on your network, leaving out hosts that have not registered their interest. Note here that an exact copy of the original message is sent to all interested parties. The original source and destination addresses are NOT changed by your router.

With that in mind, we can start to write some code.

IPAddress multicastGroup = IPAddress.Parse("239.1.2.3");
int port = 43;
IPEndPoint channel = new IPEndPoint(multicastGroup, port);
UdpClient client = new UdpClient(43);
client.JoinMulticastGroup(multicastGroup);

In the above, I set up a few variables or things like the multicast address that we are going to join, the port number, and so on. I pass the port number to the new UdpClient I create, letting it know that we are interested in messages sent to that port. I also create a variable called channel, which we will be using later.

Next up, we need to figure out a way to send a message. Unfortunately, the UdpClient class only supports sends arrays of bytes, so we will be have to convert anything we want to send to and from a byte array. Thankfully though this isn't too tough:

string data = "1 2, 1 2, Testing!";
byte[] payload = Encoding.UTF8.GetBytes(data);
string message = Encoding.UTF8.GetString(payload);

The above converts a simple string to and from a byte[] array. If you're interested, you can also serialise and deserialise C♯ objects to and from a byte[] array by using Binary Serialisation. Anyway, we can now write a method to send a message across the network. Here's what I came up with:

private static void Send(string data)
{
    Console.WriteLine("Sending '{0}' to {1}.", data, destination);
    byte[] payload = Encoding.UTF8.GetBytes(data);
    Send(payload);
}
private static void Send(byte[] payload)
{
    client.Send(payload, payload.Length, channel);
}

Here I've defined a method to send stuff across the network for me. I've added an overload, too, which automatically converts string into byte[] arrays for me.

Putting the above together will result in a multicast message being sent across the network. This won't do us much good though unless we can also receive messages from the network too. Let's fix that:

public static async Task Listen()
{
    while(true)
    {
        UdpReceiveResult result = await client.ReceiveAsync();
        string message = Encoding.UTF8.GetString(result.Buffer);
        Console.WriteLine("{0}: {1}", result.RemoteEndPoint, message);
    }
}

You might not have seen (or heard of) asynchronous C# before, but basically it's a ways of doing another thing whilst you are waiting for one thing to complete. Dot net perls have a good tutorial on the subject if you want to read up on it.

For now though, here's how you call an asynchronous method from a synchronous one (like the Main() method since that once can't be async apparently):

Task.Run(() => Listen).Wait();

If you run the above in one program while sending a message in another, you should see something appear in the console of the listener. If not, your computer may not be configured to receive multicast messages that were sent from itself. In this case try running the listener on a different machine to the sender. In theory you should be able to run the listener on as many hosts on your local network as you want and they should all receive the same message.

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