Re: multicast problem on java
Re: multicast problem on java
- Subject: Re: multicast problem on java
- From: Lasaro Camargos <email@hidden>
- Date: Tue, 11 Apr 2006 09:33:50 +0200
Thanks for the answer, Josh.
I worked around the problem by having the multicast socket bound and
using another udp socket for sending packets. Let me know if anybody
has a better way of doing this.
Lásaro.
On Mar 27, 2006, at 11:08 PM, Josh Graessley wrote:
This is expected behavior. The network stack filters traffic based
on ports and addresses, not multicast groups. When you join a group
it tells the ethernet interface to start receiving those multicasts
and sends out an IGMP request to join the group. When those packets
are received, any sockets bound to the destination address or the
any address and the specific port will receive those packets.
Don't solve this problem by binding to multicast address. You'll
run in to another problem. When sending on a socket bound to a
multicast address, the multicast address will be used as the source
address. This will either cause the packet to get dropped in the
stack or sent with a multicast source where it will be dropped or
ignored shortly thereafter. This is a side effect of the way that
multicast was hacked in to the BSD stack.
The best solution is to get the destination address for the packet.
This is normally done using an ioctl to indicate you want that
information and recvmsg to get that information through the control
data. I have no idea how you do that stuff from Java.
-josh
On Mar 25, 2006, at 2:09 PM, Lasaro Camargos wrote:
Hi everybody.
I am finding a weird behavior playing with multicast in java 1.5.
I find the same behavior on my PowerBook and on an XServer.
The problem is the following: there are two multicast groups M1
and M2. Both bind on the same port P. When I multicast something
to M1:P, M2:P also gets it. Can you tell me whether this is the
expected behavior or a bug?
The code below reproduces the behavior.
Thanks a lot.
Lásaro.
package test;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.MulticastSocket;
public class TestMulticast extends Thread{
String groupName1 = "239.0.0.1",
groupName2 = "239.0.0.2";
MulticastSocket msocket1,
msocket2;
DatagramSocket dsocket;
int port = 8000;
private InetAddress group1,group2;
DatagramPacket spacket,rpacket1,rpacket2;
private byte[] outbuf, inbuf1, inbuf2;
public void run() {
try {
//joining group.
msocket1 = new MulticastSocket(port);
msocket2 = new MulticastSocket(port);
group1 = InetAddress.getByName(groupName1);
group2 = InetAddress.getByName(groupName2);
msocket1.joinGroup(group1);
msocket2.joinGroup(group2);
//multicasting
outbuf = "TO BE MULTICAST TO JUST ONE GROUP".getBytes();
dsocket = new DatagramSocket();
InetAddress groupAddr = InetAddress.getByName
(groupName1);
spacket = new DatagramPacket(outbuf, outbuf.length,
groupAddr, port);
dsocket.send(spacket);
//receiving
inbuf1 = new byte[outbuf.length];
inbuf2 = new byte[outbuf.length];
rpacket1 = new DatagramPacket(inbuf1, inbuf1.length);
rpacket2 = new DatagramPacket(inbuf2, inbuf2.length);
// Wait for packet
msocket1.receive(rpacket1);
System.out.println("Received 1:" + new String(inbuf1));
msocket2.receive(rpacket2);
System.out.println("Received 2:" + new String(inbuf2));
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
new TestMulticast().start();
}
}
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