IP Multicast

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IP Multicast

• Stephen Li
• CSC457 Computer Networks
• University of Rochester

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Outline

• Introduction of multicast
• Model of a Host IP Implementation
• Intra-Domain Multicast
• Inter-Domain Multicast
• Summary

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What is multicast?

• One to- many, many to- many data transmission

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Why multicast?

• Videoconferencing

LAN 1
internet
router
LAN 3
LAN 2
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Address for Multicast

• Ethernet multicast address
• First bit is set 1
• IP multicast address (Host group address)
• Class D (from 224.0.0.0 to 239.255.255.255)
• 224.0.0.0 is not assigned to any group
• 224.0.0.1 is assigned to the permanent group of
  all IP hosts
• IP multicast address is handled by IGMP (Internet
  Group Management Protocol)

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Model of a Host IP Implementation
Upper-Layer Protocol Modules
IP Service Interface
ICMP
IGMP
IP Module
Local Network Service Interface
ARP
Local Network Modules (e.g Ethernet)
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Extension to IP Service Interface

• Basic functionality
• Multicast IP datagrams are sent with specified IP
  host group address rather than an individual IP
  address.
• Desired functionalities
• Provides a way for upper layer to specify IP
  time-to-live of an outgoing multicast datagram
• Provides a way for upper layer to identify which
  network interface is used for multicast
  transmission

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Extension to Ethernet Local Network Module

• Maps IP host group address to Ethernet multicast
  addresses
• Low-order 23-bits of IP address to low-order
  23-bits of Ethernet address
• It is possible that multiple host group addresses
  are mapped to one Ethernet address since the
  number of significant bits of IP host group
  address is 28

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Extension to Receiver

• Extension to IP service interface
• An upper-layer protocol must ask the IP module to
  join that group
• It is permissible to join the same group on more
  than interface
• It is also permissible for more than one
  upper-layer protocol to request membership in the
  same group

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Reverse-Path Multicast(Dense Mode)

• Source broadcasts each packet on its local
  network
• Each router that receives a packet performs a
  Reverse Path Forwarding(RPF) check
• Leaf router checks if it knows of any group
  member on its attached subnets by IGMP queries
• Either forward or generate a prune message
• Prune message are forwarded back toward source

router
Leaf router
source
Local Network
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Protocol Independent Multicast - Sparse Mode
(PIM-SM)

• RP Rendezvous Point
• Each group has a RP
• RP discovery is done using a bootstrap protocol
• Robustness When the primary RP goes down,
  bootstrap protocol can select an alternate RP
• Receivers send explicit join message to RP
• Each source sends multicast data packets
  encapsulated in unicast packets, to RP

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Example of PIM-SM
RP
Join
R4
R3
R2
R5
R1
Shared tree
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Comparison between sparse mode and dense mode

• Advantage of sparse mode
• Sparse mode has better scalability in terms of
  routing state
• Sparse mode is more efficient with the explicit
  join message than tunnel establishment
• Disadvantage of sparse mode
• RP can be a single point of failure
• RP can become a host spot for multicast traffic
• Non-optimality may exist in the multicast tree

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Multicast Backbone (Mbone)

• Mbone is a set of multicast-capable
  hosts/networks connected by tunnels
• Mbone has no central management
• New sites can be connected anywhere (flat
  topology)

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Example of MBone
Multicast-capable
Multicast-capable
Internetwork
Network2
Network1
R2
R1
10.0.0.1
Destination 2.x
Destination10.0.01 Destination2.x
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Existing Problems in Intra-Domain Protocol

• Scalability
• Most of the routes had long prefixes, which meant
  that very few hosts could be represented in each
  routing table entry
• How to apply route aggregation and hierarchical
  routing to mulitcast
• Manageability
• The utilization of tunnel may be redundant
• Domain boundaries

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Inter-Domain Multicast

• MBGP Multiprotocol Border Gateway Protocol
• Review of BGP
• Autonomous System (AS) boundaries between
  Internet domain
• ASes are commonly managed by different
  organizations
• Entities in one AS are not trusted by entities in
  another AS
• BGP provides policy control among ASes by
  advertising a complete path

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Example of BGP
128.96
AS4
AS5
AS2
192.4.3
AS1
AS6
AS3
AS7
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MBGP(cont)

• The basic of MBGP
• Subsequent Address Family Identifier fields are
  used to specify the cases unicast, multicast,
  unicast/multicast
• Each router only needs to know the topology of
  its own domain and paths to reach each of other
  domain

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Example of MBGP
Intra-domain cloud
Intra-domain cloud
Intra-domain cloud
Intra-domain cloud
Multicast-capable Border router (running MBGP)
Unicast-only Border router (running BGP)
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Summary

• Introduction of Multicast
• Host Extension to IP multicast
• Two examples of Intra-Domain Protocol for
  multicast dense and sparse modes
• Existing deployment MBone
• One example of Inter-Domain Protocol for multicast