The
Multicast method uses the same idea as broadcast - connectionless
messaging, to many nodes - UDP transport of datagrams.The Multicast is a
standard communication method and is supported in most network
implementations (FDDI, Token Ring, Ethernet, etc.)
The Multicast Datagram
The only allowed address range for multicast groups is Class D -
224.0.0.0 to 239.255.255.255. the first address is for communicating
with routers (224.0.0.1), andall the NICs (Network Interface card)
automatically joins this group.
The Multicast Workflow
One of the main ideas behind multicast is to reduce the load on
hosts which are not interested in information, meaning that the packet
coming from the network is discarded in a very early begining of the
protocol stack, if possible, then on the interface card (NIC):Protocol
Stack:UDPIPDevice DriverInterface Cardthis doesn't happen in broadcast,
since there filtering occours only at the UDP layer.Another another
advantage is the fact that routers relay multicast traffic to other
routers so all the hosts in the network which Joined a specific group
would recieve this data.This is where IGMP (Internet Group Managment
Protocol) comes in:If the multicast is implemented on a single LAN,
nothing is done with the IGMP reports, but if there are routers in the
network, then they start mapping the nodes which belong togroup and
start monitoring groups to make sure, there are still nodes which belong
to any group.Every couple of minutes the router sends a query message
to each group to see if there are still nodes in it, if there is a
response from anyone in the group, then nothing is happened, and the
router keeps transmitting multicast packets into the network.else, the
router will throw the incoming multicast packets, since no nodes exists
in the group to which the packet is directed.
The IGMP Message within an IP
datagram:|----------------------|--------------||IP Header (20 bytes) |
IGMP Message||----------------------|--------------|Format of the
fields in IGMP
message:|--------------|---------------|------------|---------------------------||4-bit
| 4-bit | |||IGMP | IGMP | Unused |
16-bit checksum ||Version (1) | type (1-2) |
|||-----------------------------------------------------------------------||32-bit
group address (IP address class D)
||-----------------------------------------------------------------------|The
IGMP version is 1.The value of type=1 is a query message sent by a
multicast routetr.The value of type=2 is a respone sent by a host.The
group address is a class D IP address:* In a query the address is set to
0* In the report, the address is set to the multicast group address
being reportedOne of the best things about Multicast packets, is the
fact the you can control the amount of hops a packet is doing between
routers, so that multicast packets only stay in thesame subnet /
network, this field is the TTL - Time to live.
Implementing The Multicast
//Socket creation, regular UDP socket
private Socket UDPSocket = new
Socket(AddressFamily.InterNetwork,SocketType.Dgram,ProtocolType.Udp);
private string Target_IP = "224.10.10.10";
private int Target_Port = 31337;
//socket initialization
public MulticastSocket()
{
//nothing should go wrong in here
try
{
//recieve data from any source
IPEndPoint LocalHostIPEnd = new
IPEndPoint(IPAddress.Any,Target_Port);
//init Socket
properties:
UDPSocket.SetSocketOption(SocketOptionLevel.Udp,SocketOptionName.NoDelay,1);
//allow for loopback testing
UDPSocket.SetSocketOption(SocketOptionLevel.Socket,SocketOptionName.ReuseAddress,1);
//extremly important to bind the
Socket before joining multicast groups
UDPSocket.Bind(LocalHostIPEnd);
//set multicast flags, sending flags - TimeToLive (TTL)
// 0 - LAN
// 1 - Single Router Hop
// 2 - Two Router Hops...
UDPSocket.SetSocketOption(SocketOptionLevel.IP,SocketOptionName.MulticastTimeToLive,
0);
//join multicast group
UDPSocket.SetSocketOption(SocketOptionLevel.IP,SocketOptionName.AddMembership,new
MulticastOption(Target_IP));
//get in waiting mode for data -
always (this doesn't halt code execution)
Recieve();
}
catch (Exception e)
{
Console.WriteLine(ex.Message + "
" + ex.StackTrace);
}
}
//client send function
public void Send(string sendData)
{
byte[] bytesToSend = Encoding.ASCII.GetBytes(sendData);
//set the target IP
IPEndPoint RemoteIPEndPoint = new
IPEndPoint(Target_IP,Target_Port);
EndPoint RemoteEndPoint = (EndPoint)RemoteIPEndPoint;
//do asynchronous send
UDPSocket.BeginSendTo(bytesToSend, 0,
bytesToSend.Length, SocketFlags.None,RemoteEndPoint,
new AsyncCallback(SendCallback), UDPSocket);
}
//executes the asynchronous send
private void SendCallback(IAsyncResult ar)
{
try
{
// Retrieve the socket from the state object.
Socket client = (Socket) ar.AsyncState;
// Complete sending the data to the remote device.
int bytesSent = client.EndSendTo(ar);
}
catch (Exception e)
{
Console.WriteLine(e.ToString());
}
}
//initial receive function - called only once
private void Recieve()
{
try
{
IPEndPoint LocalIPEndPoint = new
IPEndPoint(IPAddress.Any ,Target_Port);
EndPoint LocalEndPoint = (EndPoint)LocalIPEndPoint;
// Create the state object.
StateObject state = new StateObject();
state.workSocket = UDPSocket;
// Begin receiving the data from the remote device.
UDPSocket.BeginReceiveFrom(state.buffer, 0,StateObject.BufferSize,
0,ref LocalEndPoint,
new AsyncCallback(ReceiveCallback), state);
}
catch (Exception e)
{
Console.WriteLine(e.ToString());
}
}
//executes the asynchronous receive - executed everytime data
is received on the port
private void ReceiveCallback( IAsyncResult ar )
{
try
{
IPEndPoint LocalIPEndPoint = new
IPEndPoint(IPAddress.Any,Target_Port);
EndPoint LocalEndPoint = (EndPoint)LocalIPEndPoint;
// Retrieve the state object and the client socket
// from the async state object.
StateObject state = (StateObject) ar.AsyncState;
Socket client = state.workSocket;
// Read data from the remote device.
int bytesRead = client.EndReceiveFrom(ar,ref LocalEndPoint);
//keep listening
client.BeginReceiveFrom(
state.buffer, 0, StateObject.BufferSize, 0,ref LocalEndPoint,
new AsyncCallback(ReceiveCallback), state);
}
catch (Exception e)
{
Console.WriteLine(e.ToString());
}
}
Using the New Socket
You can find the test project and all source codes here**********************Interesting
Facts:1. A multicast packet is actually a UDP packet2. multicast
address range is from 224.0.0.0 to 239.255.255.2553. multicast can be
implemented on a simple LAN with no routers, just TCP/IP4. multicast is
much better than broadcast5. 224.0.0.1 is a permanent address assisgned
by the IANA, which means "all hosts on this subnet"6. 224.0.0.2 is also a
permanent address which means "all routers in a this subnet"7.
224.0.1.1 is for NTP - Network Time Protocol8. 224.0.0.4 is used by
DVMRP (Distance Vector Multicast Routing Protocol) - used for multicast
routing ;)9. Don't use the XP Firewall option protection on the NIC -
this won't allow you to get broadcast / multicast packets from the
network**********************Sources:TCP/IP Ilustrated, Volume 1 - W.
Richard Stevens (2000)MSDN
This article was originally written by nashcontrol |
