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# Class java.net.MulticastSocket

    java.lang.Object
       |
       +----java.net.DatagramSocket
               |
               +----java.net.MulticastSocket

public class MulticastSocket
extends DatagramSocket

The multicast datagram socket class is useful for sending and receiving IP multicast packets. A MulticastSocket is a (UDP) DatagramSocket, with additional capabilities for joining "groups" of other multicast hosts on the internet.

A multicast group is specified by a class D IP address, those in the range `224.0.0.1` to `239.255.255.255`, inclusive, and by a standard UDP port number. One would join a multicast group by first creating a MulticastSocket with the desired port, then invoking the `joinGroup(InetAddress groupAddr)` method:

     // join a Multicast group and send the group salutations
     ...
     byte[] msg = {'H', 'e', 'l', 'l', 'o'};
     InetAddress group = InetAddress.getByName("228.5.6.7");
     MulticastSocket s = new MulticastSocket(6789);
     s.joinGroup(group);
     DatagramPacket hi = new DatagramPacket(msg, msg.length,
                                 group, 6789);
     s.send(hi);
     // get their responses!
     byte[] buf = new byte[1000];
     DatagramPacket recv = new DatagramPacket(buf, buf.length);
     s.receive(recv);
     ...
     // OK, I'm done talking - leave the group...
     s.leaveGroup(group);

When one sends a message to a multicast group, all subscribing recipients to that host and port receive the message (within the time-to-live range of the packet, see below). The socket needn't be a member of the multicast group to send messages to it.

When a socket subscribes to a multicast group/port, it receives datagrams sent by other hosts to the group/port, as do all other members of the group and port. A socket relinquishes membership in a group by the leaveGroup(InetAddress addr) method. Multiple MulticastSocket's may subscribe to a multicast group and port concurrently, and they will all receive group datagrams.

Currently applets are not allowed ot use multicast sockets.

##

MulticastSocket()
  Create a multicast socket.
MulticastSocket(int)
  Create a multicast socket and bind it to a specific port.

##

getInterface()
  Retrieve the address of the network interface used for multicast packets.
getTTL()
  Get the default time-to-live for multicast packets sent out on the socket.
joinGroup(InetAddress)
  Joins a multicast group.
leaveGroup(InetAddress)
  Leave a multicast group.
send(DatagramPacket, byte)
  Sends a datagram packet to the destination, with a TTL (time- to-live) other than the default for the socket.
setInterface(InetAddress)
  Set the outgoing network interface for multicast packets on this socket, to other than the system default.
setTTL(byte)
  Set the default time-to-live for multicast packets sent out on this socket.

##

MulticastSocket

     public MulticastSocket() throws IOException

  Create a multicast socket.

MulticastSocket

     public MulticastSocket(int port) throws IOException

  Create a multicast socket and bind it to a specific port.

  Parameters:
    local - port to use

##

setTTL

     public void setTTL(byte ttl) throws IOException

  Set the default time-to-live for multicast packets sent out on this socket. The TTL sets the IP time-to-live for `DatagramPackets` sent to a MulticastGroup, which specifies how many "hops" that the packet will be forwarded on the network before it expires.

  The ttl is an unsigned 8-bit quantity, and so must be in the range `0 < ttl <= 0xFF` .

  Parameters:
    ttl - the time-to-live

getTTL

     public byte getTTL() throws IOException

  Get the default time-to-live for multicast packets sent out on the socket.

joinGroup

     public void joinGroup(InetAddress mcastaddr) throws IOException

  Joins a multicast group.

  Parameters:
    mcastaddr - is the multicast address to join
  Throws: IOException
    is raised if there is an error joining or when address is not a multicast address.

leaveGroup

     public void leaveGroup(InetAddress mcastaddr) throws IOException

  Leave a multicast group.

  Parameters:
    mcastaddr - is the multicast address to leave
  Throws: IOException
    is raised if there is an error leaving or when address is not a multicast address.

setInterface

     public void setInterface(InetAddress inf) throws SocketException

  Set the outgoing network interface for multicast packets on this socket, to other than the system default. Useful for multihomed hosts.

getInterface

     public InetAddress getInterface() throws SocketException

  Retrieve the address of the network interface used for multicast packets.

send

     public synchronized void send(DatagramPacket p,
                                   byte ttl) throws IOException

  Sends a datagram packet to the destination, with a TTL (time- to-live) other than the default for the socket. This method need only be used in instances where a particular TTL is desired; otherwise it is preferable to set a TTL once on the socket, and use that default TTL for all packets. This method does not alter the default TTL for the socket.

  Parameters:
    p - is the packet to be sent. The packet should contain the destination multicast ip address and the data to be sent. One does not need to be the member of the group to send packets to a destination multicast address.
    ttl - optional time to live for multicast packet. default ttl is 1.
  Throws: IOException
    is raised if an error occurs i.e error while setting ttl.
  See Also:
    send, receive

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