1. Block 14 Group Communication (Multicast)
Jin Sa
2018/7/5
Client-server programming

2. Client-server programming
Outline
Concepts
Java basic multicast implementation
Program examples
2018/7/5
Client-server programming

3. Client-server programming
Unicast vs. Multicast
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Client-server programming

4. Multicast applications
Multicast allows the efficient distribution of information between a single multicast source and multiple receivers.
Examples of multicast applications are audio/video services such as IPTV and conferencing services such as NetMeeting
2018/7/5
Client-server programming

5. Client-server programming
Multicast group
A set of processes form a group, called a multicast group.
The processes can be on different hosts on different networks.
Each process in a group can send and receive message. A message sent to the group can be received by each participating process in the group.
2018/7/5
Client-server programming

6. Client-server programming
Multicast operations
Primitive operations: join, leave, send and receive
Note that joining a group means that the process is entitled to receive a message sent to the group. It still needs to issue a receive operation in order to receive messages.
2018/7/5
Client-server programming

7. IP Multicast addresses - 1
Instead of a single process, a multicast datagram is meant to be received by all the processes that are currently members of a specific multicast group. Hence each multicast datagram needs to be addressed to a multicast group instead of an individual process.
A multicast address is the shared address of a group.
A multicast group is specified by
a multicast IP address, and
a port number.
2018/7/5
Client-server programming

8. IP Multicast addresses - 2
The multicast addresses are in the range of to
(The address is reserved and should not be used by any application. )
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Client-server programming

9. The Java Basic Multicast API -1
At the transport layer, the basic multicast supported by Java is an extension of UDP (the User Datagram Protocol)
Datagram - recap
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Client-server programming

10. The Java Basic Multicast API - 2
In addition to the classes provided for datagram, Java provides the MulticastSocket class
A MulticastSocket is a DatagramSocket, with additional capabilities for joining and leaving a multicast group.
2018/7/5
Client-server programming

11. Implementing multicast operations in Java
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Client-server programming

12. Joining a multicast group
// join a Multicast group
// at port 3456 and
// IP address
InetAddress group =
InetAddress.getByName(" "); MulticastSocket s =
new MulticastSocket(3456); s.joinGroup(group);  
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Client-server programming

13. Sending message to a multicast group
InetAddress group =
InetAddress.getByName(" ");
MulticastSocket s =
new MulticastSocket(3456);
String msg = "a multicast message."; DatagramPacket hi =
new DatagramPacket(msg.getBytes( ), msg.length( ),group, 3456);
s.send(hi);
2018/7/5
Client-server programming

14. Receiving messages sent to a multicast group
InetAddress group=
InetAddress.getByName(" ");
MulticastSocket s=new MulticastSocket(3456);
s.joinGroup(group);
byte[] buf = new byte[1000];
DatagramPacket recv =
new DatagramPacket(buf,buf.length);
s.receive(recv);
2018/7/5
Client-server programming

15. Leaving a multicast group
MulticastSocket s =
new MulticastSocket(3456);
… …
s.leaveGroup(group);
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Client-server programming

16. Time-To-Live (TTL) for Multicast Packets -1
Need to propagate a multicast message from a host to a neighboring host delivering the message to all the participants.
Uses the Time To Live (ttl) parameter to decide how "far" from a sending host a given multicast packet should be forwarded.
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Client-server programming

17. Time-To-Live (TTL) for Multicast Packets - 2
The time-to-live (ttl) parameter limits the count of network links or hops that the packet will be forwarded on the network.
The value for ttl can range from 0 to 255. The larger the ttl, the greater the distance a data packet will travel.
2018/7/5
Client-server programming

18. Time-To-Live (TTL) for Multicast Packets - 3
  MulticastSocket s =
new MulticastSocket(3456); s.setTimeToLive(1);
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Client-server programming

19. Time-To-Live (TTL) for Multicast Packets - 4
The recommended ttl settings are:
0 localhost
1 local network
32 same site
64 same region
128 same continent
255 unrestricted, i.e. the world
2018/7/5
Client-server programming

20. Multicast program examples
Example1Sender
Example1Receiver
Example 2
Student activity
Example3
OneLinerChatroom
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Client-server programming

21. Client-server programming
Example1
Example1Sender
Set a multicast group address
Create a multicast socket on a port
Create a datagram packet to be sent to the group
Send the datagram packet using the multicast socket
Example1Receiver
Set the same multicast group address
Create a multicast socket with the same port number
Join the group
Create a datagram packet as a placeholder
Issue a receive method to receive a data packet.
To run: (needs internet)
run Example1Receiver several times to have a few participants in the group (internet on!!!)
Run Example1Sender
Source code on E:clientserverprogramming0708/multicast/MultiExample1sender and MultiExample1receiver
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Client-server programming

22. Fragment of code for Example1Sender (hand out)
InetAddress group =
InetAddress.getByName(" ");
MulticastSocket s =
new MulticastSocket(3456);
String msg="Hi everyone!";
DatagramPacket packet =
new DatagramPacket(msg.getBytes(),
msg.length(),
group,3456);
s.send(packet);
2018/7/5
Client-server programming

23. Fragment of code for Example1Receive (hand out)
InetAddress group =
InetAddress.getByName(" ");
MulticastSocket s =
new MulticastSocket(3456);
s.joinGroup(group);
byte[] buf = new byte[100];
DatagramPacket recv =
new DatagramPacket(buf, buf.length);
s.receive(recv);
System.out.println(new String(recv.getData()));
2018/7/5
Client-server programming

24. Client-server programming
Student activity
Example1Sender_loop
Set a multicast group address
Create a multicast socket on a port
Loop
Get message from scanner
Create a datagram packet containing the message to send to group
Send the datagram packet using the multicast socket
Example1Receiver_loop
Set the same multicast group address
Create a multicast socket with the same port number
Join the group
Create a datagram packet as a placeholder
Issue a receive method to receive a data packet.
2018/7/5
Client-server programming

25. Student activity: Example1Receiver_loop_1
public class Example1Receiver_loop_1 {
public static void main(String[] args) {
try {
System.out.println("Enter the process name : ");
Scanner in = new Scanner(System.in);
String processName = in.nextLine();
InetAddress group = InetAddress.getByName(" ");
MulticastSocket s = new MulticastSocket(3456);
s.joinGroup(group);
byte[] buf = new byte[100];
DatagramPacket recv = new DatagramPacket(buf, buf.length);
// see the next slide
// Loop
// * Issue a receive method to receive a data packet
// * display message. }
}// end main
}// end class
Set up and join a Multicast group
Set up DatagramPacket for receiving message
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Client-server programming

26. Student activity: Example1Receiver_loop_1
public class Example1Receiver_loop_1 {
public static void main(String[] args) {
// see the previous slide
// * Set a multicast group address
// * Create a multicast socket on a port
int count = 0;
while (count < 10) {
s.receive(recv);
System.out.println(new String(recv.getData()));
System.out.println("received from:" +
recv.getAddress() +
" and port no: " + recv.getPort()); }
s.close();
} catch (Exception ex) {
ex.printStackTrace();
}// end main
}// end class
Loop
Issue a receive method to receive a data packet.
Display message received
2018/7/5
Client-server programming

27. Student activity: Example1Sender_loop_1
public class Example1Sender_loop_1 {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
try {
InetAddress group = InetAddress.getByName(" ");
MulticastSocket s = new MulticastSocket(3456);
// See the next slide
// Loop
// * Get message from scanner
// * Create a datagram packet containing the message to send to group
// * Send the datagram packet using the multicast socket }
s.close();
} catch (Exception ex) { ex.printStackTrace(); }
}// end main
}// end class
Create a Multicast group
2018/7/5
Client-server programming

28. Student activity: Example1Sender_loop_1
public class Example1Sender_loop_1 {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
try {
// See the previous slide
// * Set a multicast group address
// * Create a multicast socket on a port
int count=1;
while (count<10) {
System.out.println("Enter the text for sendingto the group: ");
String msg = scan.nextLine();
DatagramPacket packet =
new DatagramPacket(msg.getBytes(),
msg.length(), group, 3456);
s.send(packet);
System.out.println(msg +" has been sent.");
count++; }
s.close();
} catch (Exception ex) { ex.printStackTrace(); }
}// end main
}// end class
Loop
Get message via keyboard.
Set up Datagram Packet containing the message
Send datagramPacket
2018/7/5
Client-server programming

29. Student activity: Example1Receiver_loop_1
public class Example1Receiver_loop_1 {
public static void main(String[] args) {
try {
System.out.println("Enter the process name : ");
Scanner in = new Scanner(System.in);
String processName = in.nextLine();
// see the next slide
// Loop
// * Issue a receive method to receive a data packet
// * display message. }
}// end main
}// end class
Set up and join a Multicast group
Set up DatagramPacket for receiving message
2018/7/5
Client-server programming

30. Student activity: Example1Receiver_loop_1
public class Example1Receiver_loop_1 {
public static void main(String[] args) {
// see the previous slide
// * Set a multicast group address
// * Create a multicast socket on a port
int count = 0;
while (count < 10) { }
s.close();
} catch (Exception ex) {
ex.printStackTrace();
}// end main
}// end class
Loop
Issue a receive method to receive a data packet.
Display message received
2018/7/5
Client-server programming

31. Student activity: Example1Sender_loop_1
public class Example1Sender_loop_1 {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
try {
// See the next slide
// Loop
// * Get message from scanner
// * Create a datagram packet containing the message to send to group
// * Send the datagram packet using the multicast socket }
s.close();
} catch (Exception ex) { ex.printStackTrace(); }
}// end main
}// end class
Create a Multicast group
2018/7/5
Client-server programming

32. Student activity: Example1Sender_loop_1
public class Example1Sender_loop_1 {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
try {
// See the previous slide
// * Set a multicast group address
// * Create a multicast socket on a port
int count=1;
while (count<10) {
System.out.println(msg +" has been sent.");
count++; }
s.close();
} catch (Exception ex) { ex.printStackTrace(); }
}// end main
}// end class
Loop
Get message via keyboard.
Set up Datagram Packet containing the message
Send datagramPacket
2018/7/5
Client-server programming

33. Exercises for the practicals
Implement example 1 on slide 21
Implement student activity on slide 24
(Option) Implement the OneLinerChatroom example on slide 34.
2018/7/5
Client-server programming

34. Example2– a very simple oneliner chatroom (Option)
Everyone in the group does the same thing:
Each person joins a group
After joining, each person can send one message to the group
After joining, each person can receive all the messages sent to the group.
2018/7/5
Client-server programming

35. Oneliner chatroom:Two class
MultiExample2SenderReceiver
Get the participant’s name
Set a multicast address and port number for the group
Create a separate thread (MultiReadThread) to receive multicast messages repeatedly from other members
Send the message “hello” once to the group by
creating a datagram packet
creating a multicast socket and issue send
MultiReadThread
Set the same multicast group address as everyone else
Create a multicast socket with the same port number
Joint the group
Repeatedly issue a receive method to receive data packets.
To Run
Run MultiExample2SenderReceiver on E: clientServerProgramming/multicast
Run the file a few times to create a few participants
Hit return when ready to say hello.
2018/7/5
Client-server programming