Multicast communications refers to one-to-many or many-to-many
What is Multicast?
Multicast allows the source to send a single copy of data, using a single address for the
entire group of recipients. Routers between the source and recipients use the group address
to route the data. The routers forward duplicate data packets wherever the path to
Component of Multicast service
There are three essential components of the IP Multicast service:
IP Multicast Addressing
IP Group Management
Multicast groups are identified by IP addresses in the range
188.8.131.52 - 184.108.40.206 (class D address)
Every host (more precisely: interface) can join and leave a multicast group dynamically
• no access control
Every IP datagram send to a multicast group is transmitted to all members of the group
• no security
• Sender does not need to be a member of the group
The IP Multicast service is unreliable . . .
IP Multicasting only supports UDP as higher layer
There is no multicast TCP !
The set of receivers for a multicast transmission is called a multicast group.
A multicast group is identified by a multicast address
A user that wants to receive multicast transmissions joins the corresponding multicast
group, and becomes a member of that group.
• After a user joins, the network builds the necessary routing paths so that the user receives
the data sent to the multicast group
Internet Group Management Protocol (IGMP)
IGMP provides three basic functions for IP multicast networks:
• JOIN: An IGMP host indicates that it wants to receive information from (“become a
member of”) a multicast group.
• LEAVE: An IGMP host indicates that it no longer wishes to receive information from a
• QUERY: An IGMP router can ask the hosts which groups they are members of. This is
done to verify a JOIN/LEAVE request or to look for error conditions.
There are 3 version of IGMP
1) IGMPv1:- Hosts can join multicast groups. There were no leave messages. Routers were
using a time-out based mechanism to discover the groups that are of no interest to the
2) IGMPv2:- Leave messages were added to the protocol. Allow group membership
termination to be quickly reported to the routing protocol, which is important for highbandwidth multicast groups.
3) IGMPv3:- Major revision of the protocol. It allows hosts to specify the list of hosts from
which they want to receive traffic from
Properties of Routing Protocol
Four of the most important features of multicast routing protocols are the
• Whether they use opt-in or opt-out routing protocols.
• Whether they use source-based or shared trees.
• The methods they use to find the upstream router.
Difference between OPT-IN &
Opt-in Protocols: Opt-in or sparse protocols are designed on the assumption that most subnets
in the network will not want any given multicast packet
Difference between OPT-IN &
In opt-out or broadcast-and-prune or dense protocols, it is initially assumed that every
router on the network wishes to receive multicast data, and data is sent to all routers.
Routers wishing to remove themselves from the multicast tree must then send a Prune
message to the upstream router.
Source Based Tree protocol
1) Source based tree protocol build a separate tree for each source that send data to
2) Router wishing to join the multicast group must specify both the source and the
group of the multicast data.
3) The advantage of sourced based tree protocol are that multicast data path are
always efficient and they benefit from a simpler configuration.
Shared Based Tree
1) Shared tree protocol build a single tree is used for all source for a multicast group.
2) The tree is rooted at some selected node called rendezvous point.
3) The root of each shared multicast tree must be selected in some manner such as pre configuration.
1) PIM-SM is opt-in multicast routing protocol.
2) PIM-SM by default used shared based trees with the trees rooted at a router called
Rendezvous point (RP) for a group.
3) Data is send to an (RP) via encapsulation in PIM control message sent by unicast.
PIM-DM is opt-out multicast routing protocol.
PIM-DM uses source-based trees to distribute data.
It assumes that the receivers for any multicast group are distributed densely.
Links on which the data is not required are removed from the tree using PIM Prune
5) PIM-DM support source based tree.