IP Multicasting in C#

Sending data to a Multicast Group

For a machine to send data to a multicast group the application must join the multicast group, define a TTL for the data and then send the information to the group. This is shown programmatically below -

Socket s=new Socket(AddressFamily.InterNetwork, 
				SocketType.Dgram, ProtocolType.Udp);

We first create a socket as if we were creating a normal unicast UDP socket.

IPAddress ip=IPAddress.Parse("224.5.6.7");

We now need to join a multicast group. Multicast IP addresses are within the Class D range of 224.0.0.0-239.255.255.255 - we can join any of these addresses but most we will use 224.5.6.7 for example purposes.

s.SetSocketOption(SocketOptionLevel.IP,
SocketOptionName.AddMembership, new MulticastOption(ip));

We now issue the join command, the socket will be a member of the multicast group 224.5.6.7 once we have joined it.

s.SetSocketOption(SocketOptionLevel.IP, 
		SocketOptionName.MulticastTimeToLive, 2);

This sets the time to live for the socket - this is very important in defining scope for the multicast data. Setting a value of 1 will mean the multicast data will not leave the local network, setting it to anything above this will allow the multicast data to pass through several routers, with each router decrementing the TTL by 1. Getting the TTL value right is important for bandwidth considerations.

IPEndPoint ipep=new IPEndPoint(ip, 4567);
s.Connect(ipep);

This creates the end point that allows us to send multicast data, we connect the socket to this end point. We are now a fully fledged member of the multicast group and can send data to it -

// This creates the letters ABCDEFGHIJ
byte[] b=new byte[10];
for(int x=0;x<b.Length;x++) b[x]=(byte)(x+65);

s.Send(b,b.Length,SocketFlags.None);

s.Close();

We have now sent the letters ABCDEFGHIJ to the multicast group 224.5.6.7 on port 4567. All applications listening to this will receive this data.

Receiving Data from a Multicast Group

Receiving data is easier to setup then to send in C#.

Socket s=new Socket(AddressFamily.InterNetwork, SocketType.Dgram,
 			ProtocolType.Udp);

We setup the socket in the same manner as we would for a unicast UDP socket.

IPEndPoint ipep=new IPEndPoint(IPAddress.Any, 4567);
s.Bind(ipep);

We create an IP endpoint for the incoming data to any IP address on port 4567 and bind that to the socket.

IPAddress ip=IPAddress.Parse("224.5.6.7");

s.SetSocketOption(SocketOptionLevel.IP,
    SocketOptionName.AddMembership,
        new MulticastOption(ip,IPAddress.Any));

The socket is added to the multicast group 224.5.6.7.

byte[] b=new byte[1024];
s.Receive(b);
string str = System.Text.Encoding.ASCII.GetString(b,0,b.Length);
Console.WriteLine(str.Trim());

We can now receive any data that is sent to this multicast group.

Additional Resources

There are many resources for IP multicasting, a selection of which are listed below -

IPMI - http://www.ipmulticast.com/
University of Southern California Multicast - http://netweb.usc.edu/multicast/
RFC1112 - http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc1112.html

References

CSCO01 - http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ipmulti.htm.