1. MULTICASTING
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2. Introduction Unicasting Multicasting Multiple Unicasting Broadcasting
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
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3. Applications of Multicasting
Access to distributed database
Information dissemination
Dissemination of news
Teleconferencing
Distance Learning
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4. 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 /4
i.e. total = 228 host addresses
Range is from to
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5. 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 , it is physical multicast address
Remaining 23 bits can be used to define a group
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6. 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 = )
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7. 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
subtracting 8 from the leftmost digit if it is greater than or equal to 8
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
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8. 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
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9. 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
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10. 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
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11. 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
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12. 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
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13. Multicast Routing Optimal Routing Unicast 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
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14. Multicast Routing contd…
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)
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15. 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
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16. 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.
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17. Multicast Routing Protocols
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18. 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.
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19. Multicast Open Shortest path First: MOSPF
An extension of the OSPF protocol that uses multicast link state routing to create source-based trees
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20. Multicast Distance Vector Routing (MDVR)
Multicast routing does not allow a router to send its routing table 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
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21. MDVR contd…
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
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22. 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
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23. 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.
Reverse Path Multicasting (RPM): To convert broadcasting to multicasting, the protocol uses two procedures, pruning and grafting.
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24. 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.
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25. 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
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26. 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
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27. 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
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28. Things to do
RIP, OSPF, BGP
IGMP
MBONE
MSDP
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