Group communication allows a process to send a single message to multiple recipients through multicast operations. It provides features like fault tolerance through replicated services, where client requests are multicast to all servers performing the same operation. While multicast has no guarantees about delivery or ordering, it enables applications like discovery of servers in spontaneous networks or propagation of event notifications. IP multicast implements group communication by allowing a sender to transmit a single packet to a multicast group specified by a class D address. Membership in multicast groups is dynamic and packets may be lost or arrive out of order.

1. GROUP
COMMUNICATION

2. MULTICAST OPERATION:
It is an operation that sends a single message from one process
to each of the members of a group of processes.
 the membership of the group is transparent to the sender.
 No guarantees about message delivery or ordering.

3. 1. Fault tolerance based on replicated services:
A replicated service consists of a group of servers. Client
requests are multicast to all the members of the group, each of
which performs an identical operation. Even when some of the
members fail, client can still be served.
2. Finding the discovery servers in spontaneous networking:
It can be used by servers and client to locate available discovery
services in order to register their interfaces.

4. 3. Better performance through replicated data:
Data are replicated to increase the performance of a service.
4. Propagation of event notifications :
Multicast to a group may be used to notify processes when
something happens.

5. An implementation of group communication.
IP multicast is built on top of the internet Protocol.
IP multicast allows the sender to transmit a single IP packet to a
set of computers that from a multicast group.
A multicast group is specified by a class D internet address
Class D network created for IP multicasting
1110 28-bit multicast address

6.  The membership of multicast groups is dynamic, allowing computers
join or leave at any time.
 IP Multicast is available only via UDP.
 Performs multicast by sending UDP datagrams with multicast address
and ordinary port numbers.

7. The following details are specific to IPV4
• Multicast routers:
IP packets can be multicast both:
1) Local network – Use the multicast capability of the
Lan E.g.: Ethernet.
2) Internet – Used multicast routers.
 TTL(time to live): used to limit the distance.
• Multicast address allocation :
It may be permanent or temporary.
 Permanent addr. In the range of 224.0.0.1 –
224.0.0.255
 Remaining are available for temp.

8. IP multicast have the same failure characteristics as UDP
datagrams.
 Omission failure.
Unreliable multicast.
It does not guarantee that a message will be delivered to any
member of a group.

9. Reliability Issues:
 Recipients may drop the message because its buffer is full.
Datagram sent from one multicast router to another may be lost.
Any process may fail.
If a multicast router fails.
Ordering issue:
 IP packets sent over an internetwork do not necessarily arrive in the
order in which they were sent.

10. Some examples of the effects of reliability and ordering :
1. Fault tolerance based on replicated services
all the servers receive request messages in the same order
as one another.
2. Finding the discovery servers in spontaneous networking:
Any process that wants to locate the discovery servers
multicasts request at periodic intervals for a time after it starts
up.
3. Better performance through replicated data:
The effect of lost messages and inconsistent ordering would
depend on the method of replication and the importance of all
replicas being totally up to date.
4. Propagation of event notifications :
The particular application determines the qualities required
of multicast.

11. End