IP multicast is a method of sending Internet Protocol (IP) datagrams to a group of interested receivers in a single transmission. It is often employed for streaming media applications on the Internet and private networks.(wikipedia)

1. IP Multicasting
SCS 3004 - Networking Technologies

2. Overview
• Introduction
• Multicast Applications
• Unicast, Broadcast and Multicast
• Multicast Addressing
• Multicast Address Mapping
• IGMP
• IGMP Packet Format
• IGMP Message Types

3. Introduction to IP Multicast
• Why multicast?
• When sending same data to multiple receivers
• Better bandwidth utilization
• Lesser host/router processing
• Receivers’ addresses unknown

4. Multicast Applications
• Many applications transmit the
same data at one time to multiple
receivers
• Broadcasts of Radio or Video
• Videoconferencing
• Shared Applications
• Advertisement, Stock, Distance
learning
• Synchronizing of distributed
database, websites

5. Unicast, Broadcast and Multicast
• Broadcast: One sender, all the others as receivers
• Unicast: One sender and one receiver
• Multicast: One sender (potentially many senders), many receivers
Unicast Broadcast Multicast

6. Unicast, Broadcast and Multicast
Host
Router
Unicast
Host
Router
Multicast
With 3 receivers, sender must
replicate the stream 3times
• Source transmits one stream of
data for n receivers
• Replication happens inside
routers and switches
• WAN links only need one copy of
the data, not n copies.

7. Multicast Addressing
• All Class D addresses are multicast addresses:

8. Multicast Addressing
• The range of addresses between 224.0.0.0 and 224.0.0.255, inclusive
• 224.0.0.1 :- signifies every system on a subnet
• 224.0.0.2 :- signifies every router on a subnet
• Multicast addresses can only be used as destination
• No ICMP error messages can be generated for multicast datagram

9. Multicast Address Mapping

10. Internet Group Management Protocol - IGMP
• How hosts tell routers about group membership
• Routers solicit group membership from directly
connected hosts
• RFC 1112 specifies first version of IGMP
• IGMP v2 and IGMP v3 enhancements
• Supported on UNIX systems, PCs, and MACs

11. IGMP Protocol
• Router: sends IGMP query at regular intervals
• Hosts belonging to a multicast group must reply to query if wishing to join or
stay in the group.
• host sends an IGMP report when it joins a multicast group (Note: multiple
processes on a host can join. A report is sent only for the first process).
• No report is sent when a process leaves a group

12. IGMP Message Types

13. IGMP Packet Format
Ethernet Header IP Header IGMP Message
14 Bytes 20 Bytes 8 Bytes
Type Max Resp. Time Checksum
Group Address
8 bits 8 bits 16 bits

14. Membership Report
Host or a router can join a group

15. Leave Report
When a host sees that no process is interested in a specific
group G, it sends a leave report.

16. General Query Message
Membership report and leave report are not enough to maintain the
membership information.

17. Reference
• [1] YouTube. “Lecture - 32 IP Multicasting”. Youtube.com. [Online]. Available
https://www.youtube.com/watch?v=TApIo_BiX6U [Accessed: 3 Aug2014].
• [2] YouTube. “Cisco Live 2014: IP Multicast Concepts, Design and Troubleshooting
(Melbourne)”. Youtube.com. [Online]. Available https://www.youtube.com/watch?v=l9RJdrdjDU8
[Accessed: 3 Aug2014].
• [3] Overview of IP Multicast
http://www.cisco.com/en/US/tech/tk828/technologies_white_paper09186a0080092942.shtml
• [4] What is IP Multicasting? Concept of IP Multicast Address Explained
http://www.thegeekstuff.com/2013/05/ip-multicasting/
• [5] IP MULTICASTING
http://www.dcs.gla.ac.uk/~lewis/networkpages/m05s08multicast.htm