1. IP Multicasting

2. Agenda
 What is Multicast ?
 Why Multicast ?
 Applications of Multicast
 Multicast Addresses
 How Multicast Works ?
 IGMP overview
 How IGMP Works ?
 IGMP v1,v2 and v3
 Multicast distribution trees (Source and Shared)
 PIM-SM,PIM-DM,PIM-SSM
 Challenges to Multicast
 Summary

3. Definition
 IP Multicast is a bandwidth-conserving
technology that reduces traffic by
simultaneously delivering a single stream
of information to thousands of corporate
recipients and homes.

4. One to Many communication
Multiple Unicast Multicast

5. Why Multicast
 Multicast is... a need. Well, at least in some scenarios. If you have
information (a lot of information, usually) that should be transmitted
to various (but usually not all) hosts over an internet, then Multicast
is the answer.
One common situation in which it is used is
when distributing real time audio and video to the set of hosts which
have joined a distributed conference.
When sending to multiple receivers:
 Better bandwidth utilization
 Fast processing
 Lower router cycles required

6. Multicast applications
 Any Applications with multiple receivers
 1-to-many or many-to-many
 Live Video distribution
 Periodic Data Delivery - "Push" technology
 stock quotes, sports scores, magazines, newspapers
 Advertisements
 Server/Web-site replication
 Reducing Network/Resource Overhead
 more efficient to establish multicast tree rather then multiple point-to-
point links
 Distributed Interactive Simulation (DIS)
 wargames
 virtual reality

7. IP Multicast addresses
 Class D addresses:
224.0.0.0 - 239.255.255.255
Addresses Description
Can only be used to specify a “Group”,
Source addresses are always unicast ip 224.0.0.1 All hosts in this subnet
addresses.
224.0.0.2 All routers in this subnet
 224.0.0.0 to 224.0.0.255 – reserved to be
used by network protocols locally, it means
they are only locally significant and packets
destined to these addresses will never get 224.0.0.5 OSPF
forwarded by router.
224.0.0.6 OSPF DR
 239.0.0.0/8 is currently an administratively
scoped address space. 224.0.0.12 DHCP server/Relay agent
 The 232.0.0.0/8 block is reserved for use by
Source-specific multicast (SSM).

8. GLOP addresses
 GLOP addresses
 Provides globally available private Class D space
 233.x.x.y/24 per AS number
 RFC2770
 How?
 AS number = 16 bits
 Insert the 16 ASN into the middle two octets of 233/8

9. How Multicast works

10. Internet Group Management
Protocol (IGMP)
 Used by the hosts to indicate that they want to
join a particular group and used by the routers to
discover if there is any intended receiver in the
subnet.
 Flavors of IGMP:
IGMP v1 , v2 and v3 { v = version }

11. How IGMP works
Multicast Router
1. Interested receivers send
Membership-Report to nearest
multicast router
2. Upon receiving Membership-Report
Multicast Router starts sending traffic
to that subnet
3. Router periodically send Membership
- Query to check if there is any
Hosts intended receiver in the subnet. If
three consecutive queries fails then
IGMP Report
router stops sending traffic to that
Multicast Traffic subnet.
IGMP Query

12. IGMP version-1
 IGMP version 1 has 2 types of messages:
1. Membership Query: Used by Router to check
whether there is any interested receiver for a
particular group.
2. Membership Report: Used by the hosts to
send their willingness to join a particular group.

13. IGMP version-2
 IGMP version 2 has 4 types of messages :
1. Membership Query – Same as v1
2. V-1 Membership report – v1 report
3. V-2 Membership report – v2 report
4. Leave group – Used by the hosts to show
their intention to leave the group.

14. IGMP version-3
 IGMP v3 is used for “source filtering” ,it means the host
will tell the router which group it wants to receive traffic
from and also which source it is looking for. It enables
routers to send traffic only from the sources for which
traffic has been requested by the receivers.
 IGMP v3 has 2 options in it :
 Include: Which source it wants to receive traffic from.
 Exclude: Which source it doesn’t want to receive traffic from.

15. Multicast distribution trees
Multicast distribution trees are the
mechanisms, which allows a control over
“how multicast traffic will flow towards the
receivers”
 There are two types of distribution trees:
1. Source distribution tree
2. Shared distribution tree

16. Source distribution tree
 In source tree the root is at the source.
Source S  Always most promising path is selected.
Hence known as Shortest Path Tree.
A B D F
Notation:
(S,G) S comma G
S Source C E
S : Source
G : Group
R Receiver
R R
Multicast traffic flow

17. Characteristic of Source tree
 Each router has to contain (S,G) entry for all the
sources hence memory requirement is high.
 Surety of best possible path from source to
receiver.
 Best suitable for the topology where receivers
are densely segregated throughout the network
and senders are less in numbers ( Eg. Radio
Broadcast).

18. Shared distribution tree
 In Shared trees there is one focal point (known as
Source S1 Rendezvous point , RP) in the middle of network and
sources forward traffic towards this and intern the RP
forwards that traffic downward to all intended receivers.
RP S2
A B D F
Notation:
(*,G) Star comma G
C E
* : Any Source
G : Group
R R

19. Characteristic of Shared tree
 Uses less resources.
 There is a possibility of sub-optimal paths from
source to receiver hence RP should be planned
carefully.
 RP itself and its location may affect the
performance.
 Best suitable for many to many applications

20. Reverse Path Forwarding
 What is RPF?
 A router forwards a multicast datagram only if received on the up stream
interface to the source (i.e. it follows the distribution tree).
 The RPF Check
 The source IP address of incoming multicast packets are checked
against a unicast routing table.
 If the datagram arrived on the interface specified in the
routing table for the source address; then the RPF check
succeeds.
 Otherwise, the RPF Check fails.

21. RPF Checks
RPF Check Fails
RPF Check Succeeds

22. Protocol Independent Multicast
 PIM is the Multicast routing protocol which is completely
independent of underlying IP protocol. Though PIM is multicast
protocol but it takes help of existing unicast routing protocol to
make RPF check and to perform multicast routing/forwarding
function and it doesn’t send or receive multicast routing updates
like other routing protocols.
 PIM Types :
1. PIM-Dense Mode
2. PIM-Sparse Mode
3. PIM-Source Specific Multicast

23. PIM-DM
 PIM-DM uses “Push” model to send multicast data down to
receivers. PIM-DM floods multicast data to all the corners of the
network and the routers which do not have any downstream receiver
prune back the unwanted traffic. This process repeats every 3
minutes.
“Flood and prune” is used by the routers to gather multicast
information.
PIM-DM only supports source trees and can not be used for
shared trees.
 Example: Company-wide announcement

24. PIM-DM Contd…
Receiver
Source
Traffic is being forwarded
to every corner of the
IGMPv2 host report
Network.
Pruned interface
Source tree
Receiver

25. PIM-SM
 PIM-SM uses “Pull” model to send multicast data down
to receivers. Traffic will be forwarded to those receivers
who explicitly request for a particular group. PIM-SM
uses shared trees to forward multicast data. Initially only
shared tree will be created and depending upon the
configuration options, traffic can remain in shared tree or
can switch to optimal source tree. PIM-SM uses the
concept of RP.
 Example: Space shuttle launch

26. PIM-SM join
RP
Source
Receiver announces desire
to join group G with igmpv2
host report – (*,G).
IGMPv2 host report
(*, G) State created from the
(*, G) Join RP to the receiver.
Shared tree
Receiver

27. PIM-SM sender registration
RP
Source
(S, G) State created only
Shared Tree along the Source Tree.
Traffic Flow
(S, G) Register (unicast)
(S, G) Join Receiver
Source Tree

28. PIM-SM traffic flow
RP
Source
Source traffic flows natively
Traffic Flow along SPT to RP.
Shared Tree From RP, traffic flows down
Source Tree the Shared Tree to Receivers.
Receiver

29. PIM-SSM
No shared trees
No register packets
No RP mapping required (no RP required!)
No RP-to-RP source discovery (MSDP)
Requires IGMP include-source list – IGMPv3
User-definable range
IANA specifies 232/8 for global SSM

30. PIM-SSM join
RP
Source
Receiver announces desire
to join group G AND source
S with an IGMPv3 include-list.
Last-hop router joins the Source
(S, G) Join Tree.
Source Tree
(S,G) state is built between the
Traffic Flow
source and the receiver.
Receiver

31. PIM-SSM traffic flow
RP
Source
Data flows down the source tree
to the receiver.
Source Tree
Traffic Flow
Receiver

32. Key challenges in Multicast
 How can a sender restrict who can
receive?
 need authentication, authorization
 encryption of data
 key distribution
 still an active area of research

33. The Soup
 IGMP : Internet Group Management Protocol is used by hosts and
routers to tell each other about group membership.
 PIM-SM : Protocol Independent Multicast-Sparse Mode is used to
propagate forwarding state between routers.
 SSM : Source Specific Multicast utilizes a subset of PIM’s functionality
to guaranty source-only trees in the 232/8 range.
 RP : An RP acts as the meeting place for sources and receivers of
multicast data. In a PIM-SM network, sources must send their traffic
to the RP. This traffic is then forwarded to receivers down a shared
distribution tree