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Multicasting Program

Multicasting Program. Multicasting (1). Described in RFC1112 (August 1989) Multicasting allows distribution of a datagram to a group of listeners who are not within the local network Routers between networks need to pass multicast datagrams. . but many do not!

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Multicasting Program

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  1. MulticastingProgram

  2. Multicasting (1) • Described in RFC1112 (August 1989) • Multicasting allows distribution of a datagram to a group of listeners who are not within the local network • Routers between networks need to pass multicast datagrams. . but many do not! • The MBONE is a way of tunneling datagrams across the Internet between islands of multicast activity

  3. Multicasting (2) • Multicasts are also sent to a special address (known as a “group”) • Multicast groups need to be agreed in advance. They are not derived from a specific network/host address • Multicast groups identify a subject area (or stream of content) rather than a specific computer or network. They are more like a TV channel number than a telephone number. • The IETF has set aside addresses from 224.0.0.1 to 239.255.255.255 specifically for multicasting

  4. Multicasting (3) • To send to (or receive from) a multicast group it is first necessary to register interest in the group • This results in an Internet Group Management Protocol (IGMP) message being sent to your router (RFCs 988/1112/2236) • Then a datagram is created, addressed to the group (and the chosen port) • Java has a specialised socket for multicasting: java.net.MulticastSocket

  5. Some multicast groups • 224.0.0.1 All hosts within local subnet • 224.0.1.7 Audio news multicast • 224.0.1.12 Video from IETF meetings • 224.0.1.20 Expts. within local subnet • 224.0.1.25 NBC Professional News • There are 268 million multicast addresses (in IPv4) with 65 thousand ports in each!

  6. java.net.MulticastSocket • Subclass of java.net.DatagramSocket • Constructed the same way • Adds some extra methods: • void joinGroup(InetAddress mcastGroup) • Enter the specifies group so that you can send or receive datagrams • void leaveGroup(InetAddress mcastGroup) • Leave a group that you previously joined • void setTimeToLive(int ttl) • Sets how far your datagrams will travel before routers ignore them • int getTimeToLive()

  7. sea.datagram.MulticastSender • Sending similar to the previous example. . • . . .but must register with the multicast group and decide the longevity • The steps involved are: • Create the MulticastSocket. • Join the multicast group(s) (on startup). • Create the DatagramPacket. • Send the packet through the socket. • Leave the multicast group (on exit).

  8. sea.datagram.MulticastSender InetAddress multicastGroup = InetAddress.getByName(multicastGroupAddr); MulticastSocket socket = new MulticastSocket(); socket.joinGroup(multicastGroup); socket.setTimeToLive(5); byte[] data = “This is the message”.getBytes(); DatagramPacket datagram = new DatagramPacket(data, data.length); datagram.setAddress(multicastGroup); datagram.setPort(9876); socket.send(datagram); socket.leaveGroup(multicastGroup);

  9. sea.datagram.MulticastReceiver • The steps are: • Create a multicast socket on an agreed port. • Join the multicast group (on startup). • Create an empty datagram. • Wait for datagram to be delivered on the socket. • Unpack and use the datagram. • Leave the multicast group (on exit).

  10. sea.datagram.MulticastReceiver InetAddress multicastGroup = InetAddress.getByName(multicastGroupAddr); MulticastSocket socket = new MulticastSocket(9876); socket.joinGroup(multicastGroup); byte[] data = new byte[1000]; DatagramPacket packet = new DatagramPacket(data, data.length); socket.receive(packet); String message = new String( packet.getData(), 0, packet.getLength()); socket.leaveGroup(multicastGroup);

  11. Useful sources of information • The Internet Engineering Task Force (IETF) which is at http://www.ietf.org -- you will be able to download RFCs from here • Multicasting try reading the HOWTO which is available from the URL: http://ftp.southcom.com.au/LDP/HOWTO/...Multicast-HOWTO.html

  12. Homework • Read through the code samples to convince yourself you understand what’s going on • Sample code can be downloaded from http://ciips.ee.uwa.edu.au/~gareth • If you can, run the examples

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