Multicasting and Multicast Routing Protocols
Chapter 12
Unicasting
R2
R1
N1 N2 N3 N4
1
1
2
2
3
3
4
N5 N6
R3 R4
Recipientis here
Source
Ethernet switch
Point-to-point WAN
Unicast router
Legend
2
Multicasting
R1
R2 R3 R4
Nomember
Nomember
Nomember
At leastsone member
At leastsone member
At leastsone member
Source
1
1 1
2
22
3
33
4
Ethernet switch
Point-to-point WAN
Multicast router
Legend
3
Multicasting versus Multiple Unicasting
D4D1 D2 D3
b. Multiple unicasting
S1
DiGi
Multicast router
Unicast destinationGroup member
Unicast router
Legend
S1
G1 G1 G1G1
a. Multicasting
4
Multicast Applications
Information Dissemination
Teleconferencing
Distance Learning
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Multicast Addresses
6
Multicast Addresses
Multicast addresses for protocol control traffic
7
Delivery of Multicast Packets at Data Link Layer
Ethernet multicast address range:01:00:5E:00:00:00 to 01:00:5E:7F:FF:FF
8
Tunneling
When network does not support multicast, multicast packet encapsulated in unicast packet and sent through the network (tunneling)
9
Internet Group Management Protocol (IGMP)
Multicast routers need to collect information about members and share it with each other
Information collected locally by multicast router connected to network (IGMP protocol)
Collected information globally propagated to other routers (multicast routing protocols)
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Internet Group Management Protocol (IGMP)
IGMP is group management protocol Helps multicast router create and update list of
members related to each router interface Provides source-specific multicast
Recipient can choose to receive multicast packets coming from list of predefined sources
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IGMP Messages
12
Membership Query Message
Sent by router to find active group members in network
Max Response Code: defines maximum time before sending report in response to query
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Three Forms of Query Messages
Router probes each neighbor to report if it is interested in any multicast group
Router probes each neighbor to report if it is still interested in specific multicast group
Router probes each neighbor to report if it is still interested in receiving packets from specific multicast group, coming from any of N sources
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Membership Report Message
15
Record Type
Each host has record for each multicast group Group record has one of two modes:
Include: List source addresses accepts packets from Exclude: List source addresses does not accept from
Report message has six record types
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IGMP at Host - Socket State
Each process (associated with socket) on host has record for each multicast group
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IGMP at Host - Interface State
Interface connecting host to network has record for each multicast group
Combines socket records with same multicast group
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IGMP at Host - Interface State
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226.14.5.2 :
228.24.21.4 :
IGMP at Router - Router States
Router maintains state information for each multicast group associated with each network interface
Forwarding (or not forwarding) received packet based on router Interface state
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IGMP Message Encapsulation
IGMP messages encapsulated in IP datagrams Destination IP address depends on message
type
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Multicast Routing
Multicast routers need to collect information about members and share it with each other
information collected by IGMP is disseminated to other routers using multicast routing protocols
In multicast routing, each router constructs shortest path tree for each group
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Unicast Routing
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Each router has table defines shortest path tree to destinations
Multicast Routing using Source-Based Tree
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Each router has one shortest path tree for each group
Multicast Routing using Group-Shared Tree
25
Only core router has shortest path tree for each group
Multicast Routing Protocols
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Multicast Link State Routing
Uses source-based tree approach Extension of unicast link state routing Node advertises group with members on the link Router creates n shortest path trees (for n groups)
using Dijkstra’s algorithm
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Multicast Open Shortest Path First (MOSPF) Protocol
Extension of OSPF Protocol Uses multicast link state routing to create source-
based trees Uses new link state update packet to associate
source with group (group-membership LSA) Router calculates shortest path trees on demand
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Multicast Distance Vector Routing
Uses source-based tree approach Uses four strategies, each built on its predecessor
Flooding Reverse Path Forwarding (RPF) Reverse Path Broadcasting (RPB) Reverse Path Multicasting (RPM)
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RPF
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Flooding broadcasts packets, but creates loops RPF eliminates loop in flooding
Problem with RPF
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RPF dose not guarantee each network receives only one copy of packet
RPF versus RPB
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RPB guarantees each destination receives only one copy of packet
RPF, RPB, and RPM
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RPM adds pruning and grafting to RPB to create multicast shortest path tree supports dynamic membership changes
Distance Vector Multicast Routing Protocol (DVMRP)
Source-based routing protocol Uses multicast distance vector routing Based on RIP
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Core-Based Tree (CBT) Protocol
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Group-shared routing protocol
CBT - Sending Multicast Packets
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Protocol Independent Multicast (PIM)
PIM Dense Mode (PIM-DM) Used in dense multicast environment Source-based routing protocol Uses RPF and pruning/grafting Operates like DVMRP
PIM Sparse Mode (PIM-SM) Used in sparse multicast environment Group-shared routing protocol Operates like CBT
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Multicast Backbone (MBONE)
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Uses logical tunneling for multicasting between noncontiguous multicast routers
MBONE
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