MULTICASTING
Network Security
Introduction
 Unicasting

One source & one destination
 Multicasting

One source & group of
destinations
 Multiple Unicasting

One source send several
packets each with different
unicast destination address
 Broadcasting

One source send packets to
all the members of a network
Network Security
Applications of Multicasting
 Access to distributed database
 Information dissemination
 Dissemination of news
 Teleconferencing
 Distance Learning
Network Security
Multicast Addresses
 It is a destination address for a group of hosts that have





joined a multicast group
A packet sent to a multicast address must be delivered to
each member of the group
Addresses in class D of IPv4 are used for multicast
communication
Addresses in classes A, B, or C are mostly used for
unicast communication
Block assigned for multicasting is 224.0.0.0/4
i.e. total 232-4 = 228 host addresses
Range is from 224.0.0.0 to 239.255.255.255
Network Security
Physical Multicast Support
 Ethernet supports physical multicast addressing
 An Ethernet physical address (MAC address) is six octets (48
bits) long
 If the first 25 bits in an Ethernet address are 00000001
00000000 01011110 0, it is physical multicast address
 Remaining 23 bits can be used to define a group
Network Security
Conversion: IP multicast address to
Ethernet address

Extract the least significant 23 bits of a class D IP address and insert
them into a multicast Ethernet physical address

Ethernet multicast physical address ranges from
01:00:5E:00:00:00 to 01:00:5E:7F:FF:FF
(01:00:5E:0 = 0000 0001 0000 0000
Network Security
0101 1110 0)
Examples
Change the multicast IP address 230.43.14.7 to an Ethernet
multicast physical address.
We write the LSB (rightmost) 23 bits of the IP address in
hexadecimal:
Change the rightmost 3 bytes to hexadecimal
b) subtracting 8 from the leftmost digit if it is greater than or equal to 8
a)
The result will be 2B:OE:07
Since leftmost digit i.e. 2 is not >= 8, so we skip the (b) part and
add the result to the starting Ethernet multicast address, which is
01:00:5E:00:00:00
Answer is:
01:00:5E:2B:0E:07
Network Security
More examples
Change the multicast IP address 238.212.24.9 to an
Ethernet multicast physical address.
The LSB (rightmost) 3 bytes in hexadecimal is D4:18:09
We need to subtract 8 from the leftmost digit, resulting in
54:18:09 (D i.e. 13 > 8)
We add the result to the Ethernet multicast starting address
Answer is:
01:00:5E:54:18:09
Network Security
Unicast Routing Protocols

A routing table can be either static or dynamic
 A static table is one with manual entries
 A dynamic table is one that is updated automatically when there is a
change somewhere in the internet

A routing protocol is a combination of rules and procedures that lets
routers in the internet inform each other of changes
 It allows routers to share whatever they know about the internet or their
neighborhood
Network Security
Distance Vector Routing
 Each node maintains a vector (table) of minimum
distances to every node
 the least-cost route between any two nodes is the route
with minimum distance
 Routing Information Protocol (RIP) is based on distance
vector routing
Network Security
Link State Routing
 each node in the domain has the entire topology of the
domain i.e. list of nodes and links, how they are
connected including type, cost (metric), and condition of
links (up or down)
 the node use Dijkstra's algorithm to build a routing table
 each node has the routing table showing least-cost node
to every other node




Creation of the states of the links by each node (LSP)
Dissemination of LSPs to every other router (flooding)
Formation of a shortest path tree for each node
Calculation of a routing table based on the shortest path
tree
 OSPF protocol is based on link state routing
Network Security
Path vector routing
 similar to that of distance vector routing
 there is one node that acts on behalf of the entire system
(speaker node)
 creates a routing table and advertises it to speaker nodes
in the neighboring systems
 only speaker nodes in each system can communicate
with each other
 Border Gateway Protocol (BGP) is based on path vector
routing
Network Security
Multicast Routing
 Optimal Routing



To define a shortest path tree to possible destinations
The root of the tree is source, and leaves are the potential destinations
Path from the root to each destination is the shortest path
 Unicast Routing


Each router has its own shortest path tree (SPT)
Each line of the routing table is a shortest path
Network Security
Multicast Routing contd…
Multicast Routing




A multicast packet may have destinations in more than one
network
If we have n groups, we may need n shortest path trees
Each involved router needs to construct a shortest path tree
for each group
Two approaches:
 Source-based trees (SBT) and Group-shared trees
(GST)
Network Security
Source-based tree approach

Each router needs to have one shortest path tree for each group
 The shortest path tree for a group defines the next hop for each network
that has loyal member(s) for that group

If the number of groups is m, each router needs to have m shortest path
trees, one for each group
Network Security
Group-shared tree approach
 There is only one designated router, called the center core, or
rendezvous router
 The core has m shortest path trees in its routing table.
 The rest of the routers in the domain have none.
Network Security
Multicast Routing Protocols
Network Security
Multicast Link State Routing








It uses the source-based tree approach
A direct extension of unicast routing
Each router creates a shortest path tree by using Dijkstra's algorithm
A node advertises every group which has any loyal member on the link.
It needs to revise the interpretation of state (i.e. what groups are active
on the link)
The information about the group comes from IGMP running on each
router
When a router receives all the LSPs (Link State Packets), it creates n
topologies from which n shortest path trees are made by using Dijkstra's
algorithm
The only problem with this protocol is the time and space needed to
create and save the many shortest path trees :- The solution is to create
the trees only when needed.
Network Security
Multicast Open Shortest path
First: MOSPF
 An extension of the OSPF protocol that uses multicast link state
routing to create source-based trees
Network Security
Multicast Distance Vector
Routing (MDVR)
 Multicast routing does not allow a router to send its routing table



1.
to its neighbors
Tables are created from scratch by using the information from
the unicast distance vector tables
MDVR uses source-based trees, but the router never actually
makes a routing table
It uses a process based on four decision-making strategies
Flooding: A router receives a packet and, without even looking
at the destination group address, sends it out from every
interlace except the one from which it was received



Every network with active members receives the packet
This is a broadcast, not a multicast
Also it creates loops; The next strategy, reverse path forwarding,
corrects this defect
Network Security
MDVR contd…
2. Reverse Path Forwarding (RPF): To prevent loops, only one
copy is forwarded; the other copies are dropped.



A router forwards only the copy that has traveled the shortest path
from the source to the router
To find this copy, RPF
uses unicast routing table
This strategy prevents loops
because there is always one
shortest path from the source
to the router
Network Security
MDVR contd…

RPF does not guarantee that each network receives only one copy as it
is not based on the destination address (a group address); forwarding
is based on the source address
 To eliminate duplication, we must define only one designated parent
router for each network.
 Reverse Path Broadcasting (RPB): It guarantees that the
packet reaches every network and that every network receives
only one copy
Network Security
MDVR contd…
RPB does not multicast the packet, it broadcasts it. That’s not efficient.
 The multicast packet must reach only those networks that have active
members for that particular group. This is RPM.
4. Reverse Path Multicasting (RPM): To convert broadcasting to
multicasting, the protocol uses two procedures, pruning and grafting.

Network Security
Distance Vector Multicast
Routing Protocol: DVMRP
 It is an implementation of multicast
distance vector routing.
 It is a source-based routing protocol,
based on RIP.
Network Security
Core-Based Tree (CBT)


A group-shared protocol
The autonomous system is divided into regions, and a core (center
router or rendezvous router) is chosen for each region.

Formation of the Tree: After the rendezvous point is selected, every
router is informed of the unicast address of the selected router.
 Each router then sends a unicast join message
 After receiving all join messages from every member of the group, a
tree is formed
Network Security
CBT contd…

Sending Multicast Packets:
 After formation of the tree, any source can send a multicast packet to all
members of the group
 It simply sends
the packet to the
rendezvous router
Network Security
Protocol Independent Multicast
(PIM)








Two independent multicast routing protocols: Protocol Independent
Multicast, Dense Mode (PIM-DM) and Protocol Independent Multicast,
Sparse Mode (PIM-SM)
Both protocols are unicast protocol- dependent
PIM-DM is used when there is a possibility that each router is involved
in multicasting (dense mode such as a LAN)
A source-based tree routing protocol that uses RPF and pruning and
grafting strategies for multicasting
It assumes that the autonomous system is using a unicast protocol
(RIP or OSPF) and each router has a table
PIM-SM is used when there is a slight possibility that each router is
involved in multicasting (sparse mode - WAN)
A group-shared tree routing protocol
It can switch from a GST strategy to a SBT strategy when necessary
Network Security
Things to do
 RIP, OSPF, BGP
 IGMP
 MBONE
 MSDP
Network Security