Overview

Multicast is the delivery of information to a group of destinations simultaneously, using the most efficient strategy to deliver messages over each link of the network only once, and create copies only when the links to the destinations split.

The word "multicast" is typically used to refer to IP Multicast, the implementation of the multicast concept on the IP routing level, where routers create optimal spanning tree distribution paths for datagrams sent to a multicast destination address in realtime. But there are also other implementations of the multicast distribution strategy listed below.
(Taken from wikipedia: http://en.wikipedia.org/wiki/Multicast).

GigaSpaces uses multicast in the following cases:

• Services use multicast to discover the Lookup Service and register their proxies.
• Clients use multicast to discover the Lookup Service and look up a matching service proxy (such as the space).
• Space cluster replication facilities use multicast when replicating data across different spaces.
• Multicast notifications use multicast to send the matching notification to the registered clients.

To enable the important capabilities above, you should enable multicast on machines running clients, spaces or services.

What should I do if I can't enable multicast? If you cannot enable multicast in your environment, you can use unicast discovery to allow services and clients to locate the Lookup Service. Space cluster replication uses unicast by default. You should use multicast replication when having more than 10 clients acting as replica spaces per target space.

Configuring Multicast on Linux

In some cases, Linux distributions do not have multicast enabled by default and the /etc/hosts file does not have the machine's IP.

Make sure the /etc/hosts has the machine's IP, together with the IP: 127.0.0.1.

127.0.0.1        localhost
192.168.10.127   Mylinux

Make sure all network machines can ping each other. You might need to list all the network machines' IPs as part of each machine's hosts file, or have a DHCP server configured.

Before running GigaSpaces, make sure your network interface supports multicast, and the appropriate routes are properly configured.

To check if your network interface supports multicast, run the following ifconfig:

$ /sbin/ifconfig -a
eth0      Link encap:Ethernet  HWaddr 00:30:48:2E:67:BA
          inet addr:192.168.10.127  Bcast:192.168.10.255  Mask:255.255.255.0
          inet6 addr: fe80::230:48ff:fe2e:67ba/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:89049707 errors:34 dropped:0 overruns:0 frame:34
          TX packets:79402911 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:24029278032 (22.3 GiB)  TX bytes:52438225932 (48.8 GiB)
          Base address:0x3000 Memory:dc300000-dc320000

eth1      Link encap:Ethernet  HWaddr 00:30:48:2E:67:BB
          inet addr:192.168.0.1  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::230:48ff:fe2e:67bb/64 Scope:Link
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
          Base address:0x3040 Memory:dc320000-dc340000

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:1366770822 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1366770822 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:319301580062 (297.3 GiB)  TX bytes:319301580062 (297.3 GiB)

sit0      Link encap:IPv6-in-IPv4
          NOARP  MTU:1480  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

If the MULTICAST attribute in the forth line of the eth0 properties is not present, it's possible that your kernel has not been compiled with multicast support. Before re-compiling the kernel, try enabling multicast on your network interface (eth0 in this case) via ifconfig:

$ /sbin/ifconfig etho multicast

To check that multicast routing is configured, run the following route command:

$ /sbin/route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.0.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1
192.168.10.0    0.0.0.0         255.255.255.0   U     0      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     0      0        0 eth1
224.0.0.0       0.0.0.0         240.0.0.0       U     0      0        0 eth0
0.0.0.0         192.168.10.1    0.0.0.0         UG    0      0        0 eth0

If the destination 224.0.0.0 entry is not present, you need to enable multicast routing.

To enable multicast routing, run the following route command as root:

$ /sbin/route -n add -net 224.0.0.0 netmask 240.0.0.0 dev eth0

On Redhat, systems you can configure this route statically via the network setup configuration tools – see: redhat - Chapter 6. Network Configuration.

Alternatively, try executing the command above as part of a startup script in /etc/rc.d/*.

For more details on Linux ifconfig and route commands, refer to: http://linux.about.com/od/commands/l/blcmdl8_ifconfi.htm http://linux.about.com/od/commands/l/blcmdl8_route.htm

tcpdump is another useful command which dump traffic on a network is tcpdump -i eth0 ip multicast

Configuring Multicast on Windows

To enable multicasting from a token ring on a Windows® 2000 workstation to any Windows 98/NT machine, set the TrFunctionalMcastAddress parameter to 0 in the Windows 2000 registry:

1. Click Start > Run on the Windows 2000 taskbar.
2. In the Open field, select or type REGEDIT.
3. Click OK. The Registry Editor window opens.
4. Click HKEY_LOCAL_Machine > SYSTEM > CurrentControlSet > Services > Tcpip > Parameters.
5. Right-click TrFunctionalMcastAddress, and click Modify. The Edit DWORD Value window opens.
6. In the Value data field, type 0.
7. Click OK to save changes and exit.
8. Close the Registry Editor.