Multicast sockets

1
Multicast sockets

• Lecture 9

2
What is Multicast Socket?

• Sends data from one host to many different hosts,
  but not to everyone
• Data goes only to clients who have expressed an
  interest by joining a particular multicast group.
• People can come and go as they please, leaving
  when the discussion no longer interests them
• Several copies of the data traverse the internet
• Load can be minimized by carefully choosing where
  the pkts are duplicated
• Internet routers handle the duplication process

3
Why multicasting.

• IP also supports broadcasting, but use of
  broadcasts is strictly limited
• Protocols require broadcast only when there is no
  alternative routers usually limit broadcast to
  the local net subnet, preventing it to reach
  the Internet.
• Few broadcasts can bring Internet to knees
• Broadcasting high-bandwidth data such as
  audio/video/text/imagesis out of question
• A single email spam that goes to millions of
  addresses is bad enough!!

4
The choice

• Technology that send data to a number of hosts
  (1000 clients) that need it
• Data is sent 1000 times
• Broadcast the data to every host on the internet
• Create static connection trees (Usenet)
• Multicasting
• Multicasting is half-way between p2p and
  broadcasting
• When a pkt is multicast it will not go to a
  single host (unicast) neither will it go to all
  hosts (broadcast), but sent to all host belonging
  to the group.

5
Uses of multicast

• Audio video applications
• Multiplayer games
• Distributed file system
• Massively parallel computing
• Multi-person conferencing
• Database replication
• Name services directory services
• Apples Rendezvous Suns Jini use IP
  multicasting to dynamically discover services on
  the local network

6
Uses cont..

• Used to implement various caching for the
  Internet which will help with the internets
  growth
• A high traffic web server can be split between a
  number of m/cs using multicast address
• Multicasting is designed to fit seamlessly into
  the internet. Challenge remains to work in
  ubiquitous routers.

7
Multicast addresses and groups

• Multicast addresses are IP addresses in the range
  224.0.0.0 to 239.255.255.255. All addresses in
  this range has 1110 as their first four bits.
• They are called class D addresses
• A multicast group is a set of internet hosts that
  share a multicast address. Any data send to the
  group is relayed to all the members of the group
• Membership in a multicast group is open. Hosts
  can enter or leave at anytime
• Groups can be either permanent or transparent
  (most are)
• To create a new multicast group, pick a random
  address from 225.0.0.0 to 239.255.255.255.
  Construct an InetAddress object for that address
  and start sending it data.

8
Special purpose multicast addresses

• allsystem.mcast.net at 224.0.0.1 is a multicast
  group to include all systems that suppory
  multicast on local net
• Experiment.mcast.net _at_224.0.1.20 is used for
  local testing
• All addresses beginning with 224.0.0.0 to
  224.0.0.255 are reserved for routing protocols

9
Multicast sockets

• Makes use of Datagram sockets
• Java.net.MulticastSocket class is a suclass of
  java.net.DatagramSocket
• To receive data that is multicast from a remote
  site
• create a MulticastSocket with the MulticastSocket
  () constructor.
• Next join a multicast group using
  MulticastSockets joinGroup () method. This
  signals the routers in the path between you and
  the server to start sending data your way tells
  the local host that it should pass your IP
  packets addresses to the multicast group.
• Security breach could take place!

10
Multicast constructors

• Uses an unused anonymous port
• try
• MulticastSocket ms new MulticastSocket ()
• //send some datagram..
• Catch (SocketException )
• System.err.println(se)
• Receives datagrams on a well-known port
• try
• MulticastSocket ms new MulticastSocket (4000)
• //receive incoming datagrams..
• Catch (SocketException )
• System.err.println(se)

11
MulticastSocket

• You can create a MulticastSocket using
  SocketAddress object. The socket address can be
  bound to a port
• try
• SocketAddress address new InetSocket Address
  (4000)
• MulticastSocket ms new MulticastSocket
  (address)
• //receive incoming datagram..
• Catch (SocketException )
• System.err.println(se)
• Socket address can be bound to a specific network
  interface on local host
• try
• SocketAddress address new InetSocket Address
  (192.125.22.124000)
• MulticastSocket ms new MulticastSocket
  (address)
• //receive incoming datagram..
• Catch (SocketException )
• System.err.println(se)

12
Multicast sockets..

• You can pass null to this constructor to create
  an unbound socket which would later be connected
  with a bind () method. Useful for setting socket
  options that can only be set before the socket is
  bound.
• try
• MulticastSocket ms new MulticastSocket (null)
• Ms.setReusAddress(false)
• SocketAddress address new InetSocketAddress
  (4000)
• Ms.bind(address)
• //receive incoming datagram..
• Catch (SocketException )
• System.err.println(se)

13
Communicating with a multicast group

• 1.Join a multicast group
• 2.Send data to the members of the group
• 3.Receive data from the group
• 4.Leave the multicast group
• MulticastSocket class has methods for 1, 2 4

14
Join the Multicast group

• Pass an InetAddress object
• St up datagramPacket as buffer abd pass it into
  this sockets receive method
• try
• MulticastSocket ms new MulticastSocket (4000)
• InetAddress is InetAddress.getByName(224.2.2.2
  )
• Ms.joinGroup(ia)
• Byte buffer new byte 8192
• While (true)
• DatagramPacket dp new DatagramPacket(buffer,
  buffer.length)
• Ms.receive(dp)
• String s new String (dp.getData(), 8859_1)
• System.out.println (s)
• catch (SocketException )
• System.err.println(se)