Multicast Communication in Java

1
Multicast Communication in Java

• CMT3332
• Lecture 7

2
Unicast vs. Multicast
3
Multicast

• The majority of network services and network
  applications use unicast for IPC
• Multicasting is useful for applications such as
• groupware
• online conferences
• interactive distance learning, and etc.
• It can also be used in replication of services
  for fault tolerance.

4
Mutlicast group

• In multicasting, a set of processes form a group,
  called a multicast group.
• Each process in a group can send and receive
  message.
• A message sent by any process in the group can be
  received by each participating process in the
  group.
• A process may also choose to leave a multicast
  group.
• In an application such as online conferencing, a
  group of processes interoperate using
  multicasting to exchange audio, video, and/or
  text data.

5
Multicast API

• There are FOUR primitive operations in
  Mutlicasting
• Join
• This operation allows a process to join a
  specific multicast group.
• A process that has joined a multicast group is a
  member of the group and is entitled to receive
  all multicast addressed to the group.
• A process can be a member of multiple multicast
  groups.

6
Multicast API (cont'd)

• Leave
• This operation allows a process to stop
  participating in a multicast group.
• A process that has left a multicast group is no
  longer a member of the group and is thereafter
  not entitled to receive any multicast addressed
  to the group.
• Send
• This operation allows a process to send a message
  to all processes currently participating in a
  multicast group.
• Receive
• This operation allows a member process to receive
  messages sent to a multicast group.

7
Reliable vs. Unreliable Multicast

• In a multicast environment, the runtime supports
  of the multicast mechanism is responsible for
  delivering the message
• A message may not be received by one or more of
  the processes at all, due to errors and/or
  failures in the network, the machines, or the
  runtime support.
• Some applications, such as video conferencing,
  can tolerate an occasional miss or misordering of
  messages
• Other applications such as database
  applications for which such anomalies are
  unacceptable.

8
Unreliable Multicast

• Such multicast system will make a good-faith
  attempt to deliver messages to each participating
  process, but the arrival of the correct message
  at each process is not guaranteed.
• Any message sent be received by zero or more
  processes.
• The message may be received by some processes.
  (In the best case, all and in the worst, none)
• Some messages may be received by some processes
  in a corrupted form

9
Reliable Multicast

• Such multicast system guarantees that each
  message is eventually delivered to each process
  in the group.
• Each message sent by a process can be assumed to
  be delivered in a non-corrupted form to all
  processes in the group eventually.

10
Ordered vs. Unordered Reliable Multicast

• An unordered reliable multicast system provides
  no guarantee on the delivery order of the
  messages.
• A FIFO (first-in-first-out) or send-order
  multicast system guarantees that the following
  delivery order of the messages
• If process P sent messages mi and mj, in that
  order, then each process in the multicast group
  will be delivered the messages mi and mj, in that
  order.
• Causal Order Multicast satisfies the following
  message delivery order
• If message mj causes the occurrence of message
  mj, then mi will be delivered to each process
  prior to mj.
• An atomic-order multicast system guarantees all
  messages to be delivered to each participant in
  the exact same order.

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Example on FIFO

• Suppose you have 2 processes in a multicast
  group.
• P1 sends messages m11 then m12
• P2 sends messages m21 then m22
• Then a FIFO multicast system can deliver the
  messages in any of the following orders
• m11-m12-m21-m22,
• m11-m21-m12-m22,
• m11-m21-m22-m12,
• m21-m11-m12-m22
• m21-m11-m22-m12
• m21-m22-m11-m12.

12
Example on Casual Order

• Suppose P1 multicasts message m1
• to which P2 replies with a multicast message m2
• another independent P3 replies to m1 with a
  multicast message m3
• The three messages now share these causal
  relationships m1 -gt m2 and m1 -gt m3.
• A causal-order multicast system can delivery
  these message in either of the following orders
• m1- m2- m3
• m1- m3- m2

13
Example on Atomic Order

• P1 sends m1 then m2.
• P1 must be delivered m1 before sending m2
• P2 replies to m1 by sending m3.
• P2 must receive m1 before m3
• P3 replies to m3 by sending m4.
• P3 must receive m3 before m4
• Any atomic delivery order must preserve the
  order m1 m3 m4
• An atomic multicast will result in the messages
  being delivered to each of the processes in one
  of the following orders
• m1 m2 m3 m4
• m1 m3 m2 m4 or
• m1 m3 m4 m2

14
The Java Basic Multicast API

• At the transport layer, the basic multicast
  supported by Java is an extension of UDP, which
  is connectionless and unreliable.
• The four major classes in multicasting
• InetAddress
• DatagramPacket
• DatagramSocket
• MulticastSocket
• The Java Reliable Multicast Service (JRM Service)
  provides the capabilities for a receiver to
  repair multicast data that are lost or damaged,
  as well as security measures to protect data
  privacy.

15
Classes for Mulitcast

• InetAddress
• Identify the IP of sender/receiver as well as a
  multicast group
• DatagramPacket
• packet of data that is sent to / received by each
  participant in a multicast group
• DatagramSocket
• a socket for a process to send or receive data
• MulticastSocket
• A MulticastSocket is a DatagramSocket, with
  additional capabilities for joining and leaving a
  multicast group.
• An object of the multicast datagram socket class
  can be used for sending and receiving IP
  multicast packets.

16
IP Multicast Addresses

• 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.
• The Java multicast API uses the Internet Protocol
  (IP) multicast addresses to identify multicast
  groups.
• A multicast group is specified by
• a class D IP address (224.0.0.0 (reserved) -
  239.255.255.255) and
• a standard UDP port number.

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IP Multicast Addresses (cont'd)

• Some of the Class D Addresses are assigned for
  different purpose.
• To avoid crashing, we will use 239... for our
  mulitcast exercises.
• In the Java API, a MulticastSocket object is
  bound to a port address such as 3456

18
Joining and Leaving a Multicast Group

• To join a multicast group
• // join a Multicast group at IP address //
  239.1.2.3 and port 3456
• InetAddress group
• InetAddress.getByName("239.1.2.3")
  MulticastSocket s new MulticastSocket(3456)
  s.joinGroup(group)
• A process may leave a multicast group by invoking
  the leaveGroup method
• s.leaveGroup(group)

19
Message Propagation

• Message propagation is often used to support
  Multicasting
• A packet is propagated from a host to a
  neighboring host so that it will eventually
  deliver the message to all the participants.
• Under some anomalous circumstances, however, it
  is possible that the algorithm which controls the
  propagation does not terminate properly,
  resulting in a packet circulating in the network
  indefinitely.
• Indefinite message propagation causes unnecessary
  overhead on the systems and the network.

20
Time To Live

• To avoid the infinite propagation of a packet, a
  time to live parameter is to be set with each
  multicast datagram.
• The time-to-live (ttl) parameter limits the count
  of network links or hops that the packet will be
  forwarded on the network.
• The recommended ttl settings are
• 0 restricts multicast to processes on the same
  host
• 1 restricts multicast to processes on the same
  subnet
• 32 restricts multicast to processes on the same
  site
• 64 restricts multicast to processes on the same
  region
• 128 restricts multicast to processes on the same
  continent
• 255 restricts multicast to unrestricted

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Sending to a Multicast Group

• String msg "This is a multicast message."
• InetAddress group
• InetAddress.getByName("239.1.2.3")
• MulticastSocket s new MulticastSocket(3456)
  
• s.joinGroup(group) // optional
• DatagramPacket hi new DatagramPacket(msg.g
  etBytes( ), msg.length( ),group, 3456)
  s.send(hi)

22
Receiving Messages Sent to a Multicast Group

• byte buf new byte1000
• InetAddress group InetAddress.getByName("239.1
  .2.3") MulticastSocket s new
  MulticastSocket(3456) s.setTimeToLive(1)
• s.joinGroup(group)
• DatagramPacket recv new DatagramPacket(buf,
  buf.length)
• s.receive(recv)