IP Multicasting

SCS 3004 - Networking Technologies

Overview

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

Introduction to IP Multicast

• Why multicast?• When sending same data to multiple receivers

• Better bandwidth utilization

• Lesser host/router processing

• Receivers’ addresses unknown

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

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

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.

Multicast Addressing

• All Class D addresses are multicast addresses:

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

Multicast Address Mapping

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

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

IGMP Message Types

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

Membership Report

Host or a router can join a group

Leave Report

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

General Query Message

Membership report and leave report are not enough to maintain the membership information.

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 Explainedhttp://www.thegeekstuff.com/2013/05/ip-multicasting/

• [5] IP MULTICASTINGhttp://www.dcs.gla.ac.uk/~lewis/networkpages/m05s08multicast.htm