This document provides a simple client/server example for setting up a multicast application in C# .NET as a well as the method in which multicasting works and why it is useful now and how, with the increasing availability of bandwidth and the increased use of streaming media, will become a definitive method of data transmission in the near future.
What is IP Multicasting
"IP Multicasting is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to thousands of corporate recipients and homes."CSCO01.
The concept behind multicasting can be thought of as the same as terrestrial and satellite based television broadcasting; there is a single satellite or transmitting mast and many receivers. This works because there is no back-channel to the source and routers along the path to receivers are able to create multiple copies of the stream to pass on to other routers and hosts along the route. The process works well as the implementation of the Internet is similar to driving from one side of the country to the other, you will follow various routes and join new ones where necessary that are en-route to your destination.
Requirements of IP Multicasting
A few things must be in place before developing multicasting applications or migrating unicast applications.
For IP multicasting to work all the routers along the path of communication must be multicast enabled. For internet multicasting, devices must transmit and be a member of the MBONE, which is basically a group of devices within a virtual network that support IP multicasting traffic. The MBONE is at best a temporary utility for all multicasting enabled, in time, the MBONE will fade away and just become another part of the Internet as more and more manufacturers produce routers that support multicasting.
The operating system networking interface must support multicasting. Berkeley Sockets, Windows Sockets 2 and Apple Macintosh Open Transport all support multicasting - I guess this is important as .NET will (hopefully) eventually be ported to all these operating systems.
An IP Multicast Application
RFC1112 "Host Extensions for IP Multicasting" (http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc1112.html) recommends a number of API calls that should be available for multicast support. These are - Join a multicast group
Leave a multicast group
Set the TTL of a multicast group
The examples included with this documentation will demonstrate each of these .NET Framework API calls.
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,
We first create a socket as if we were creating a normal unicast UDP socket.
We now need to join a multicast group. Multicast IP addresses are within the Class D range of 18.104.22.168-22.214.171.124 - we can join any of these addresses but most we will use 126.96.36.199 for example purposes.
SocketOptionName.AddMembership, new MulticastOption(ip));
We now issue the join command, the socket will be a member of the multicast group 188.8.131.52 once we have joined it.
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);
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 -
byte b=new byte;
for(int x=0;x<b.Length;x++) b[x]=(byte)(x+65);
We have now sent the letters ABCDEFGHIJ to the multicast group 184.108.40.206 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,
We setup the socket in the same manner as we would for a unicast UDP socket.
IPEndPoint ipep=new IPEndPoint(IPAddress.Any, 4567);
We create an IP endpoint for the incoming data to any IP address on port 4567 and bind that to the socket.
The socket is added to the multicast group 220.127.116.11.
byte b=new byte;
string str = System.Text.Encoding.ASCII.GetString(b,0,b.Length);
We can now receive any data that is sent to this multicast group.
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
CSCO01 - http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ipmulti.htm.