Multicasting

Multicast is the protocol which allow nodes inside to a cluster to communicate without knowing each other.

You can think of multicast of a radio or a TV channel, only those who are tuned received the information.

Communication between nodes is provided by JGroups, which is library for multicast communication.

What is multicasting?
There are three types of communication between computers in a network:
Unicast – one computer talks directly to another computer
Broadcast – one computer talks to all computers
Multicast – one computer talks to a select group of others

In a conventional Ethernet network, most Internet Protocol (IP) packets are sent using unicast
(host-to-host) transmission. Every computer in a network can transmit and receive packets,
which in unicast transmission are labelled with the address of the receiving computer. Each
computer listens to all the other packets that are sent in the network and looks for packets
that are addressed to itself. When a computer encounters a packet that is addressed to itself, it
interrupts the processor and hands the packet to the operating system to process.
Unicasting is great for communicating directly with one or a few other computers. However if
you want to communicate with a number of computers, unicasting becomes inefficient because
a copy of each packet must be sent to every receiving unicast address. Unicasting uses up
bandwidth fast, especially when sending large multimedia files, which already take up a lot of

Unicast transmission sends a separate stream of data to each receiver, while multicast transmission sends
one stream of data that is separated as it passes through the routers and sent on to the receivers
What are the benefits of multicasting?
Multicasting optimises the performance of your network. Because only one multicast data stream
is sent out, multicasting preserves bandwidth on your network and eliminates traffic redundancy. In
contrast, the unicast environment sends out a separate copy of the data to each receiver.
Multicasting also provides enhanced efficiency by controlling the traffic on your network and
reducing load on network devices.The clients on your network are able to decide whether or
not to listen to a multicast address, so packets are only sent to where they are required.
In addition, multicasting is scalable across different sized networks, but is particularly suited to
WAN environments. It gives people in different locations access to streaming data files, like a
video, film or live presentation without taking up excessive bandwidth or broadcasting the data
to all users on the network.

B. How do Multicast Addresses Work?
Because multicast addresses identify a transmission session rather than a specific physical
destination or host, all of the receivers in a multicast group are identified by a single IP address.
This section outlines how multicast IP and MAC addresses are structured.
How are IPv4 multicast addresses organised?
IPv4 multicasting uses class D addresses. A class D address starts with 1110 higher order bits in
the first octet, followed by a 28-bit group address.The last 28 bits of a class D address are
unstructured, unlike the class A, B and C IP addresses.These 28 bits identify the multicast group
identity, which is a single address in the range of 224.0.0.0 to 239.255.255.255.
Some IPv4 multicast addresses are reserved for particular purposes.These addresses are
assigned by the Internet Assigned Numbers Authority (IANA).Table 1 outlines some of the
well-known IPv4 multicast addresses.