An Introduction to IP Multicast

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An Introduction to IP Multicast
2
The Problem

• Its election night 2056 at RepubliCrat National
  Headquarters. Results are pouring in from all
  over the country for aggregation on a CORBA.NET
  server. By 7pm, 50 million voters have activated
  their Election Results Page using an ASP.NET
  client on their PC, and all 50 million clients
  want instantaneous updates from that single
  server

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Presentation Overview

• This presentation provides a technical
  introduction to IP Multicast concepts and
  technical features.
• It discusses the requirements for IP Multicast
  delivery, addressing, host group management, and
  some approaches to multicast routing.

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IP Multicast Background

• IP Multicast is a bandwidth-conserving technology
  that reduces network traffic
• Simultaneously deliver a single stream of
  information to thousands of recipients
• Corporate and homes
• It does not add any additional burden on the
  source or receivers while using the least network
  bandwidth

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IP Multicast Background, contd.

• Multicast packets are replicated in the network
  by routers enabled with Protocol Independent
  Multicast (PIM) and other supporting multicast
  protocols
• This configuration is the most efficient delivery
  of data to multiple receivers

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Suitable Applications

• Video conferencing
• Corporate communications
• Distance learning
• Stock quotes
• Software distribution
• News

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Three forms of casting

• Unicasting point-to-point

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Three forms of casting

• Unicasting point-to-point
• Broadcasting everyone on a subnet

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Three forms of casting

• Unicasting point-to-point
• Broadcasting everyone on a subnet
• Multicasting one-to-many or many-to-many

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The Multicast Concept

• Receivers join a particular multicast host group

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The Multicast Concept

• Receivers join a particular multicast host group
• Traffic is delivered to all members of that group
  by the network infrastructure

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The Multicast Concept

• Receivers join a particular multicast host group
• Traffic is delivered to all members of that group
  by the network infrastructure
• The sender does not need to maintain a list of
  receivers

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The Multicast Concept

• Receivers join a particular multicast host group
• Traffic is delivered to all members of that group
  by the network infrastructure
• The sender does not need to maintain a list of
  receivers
• Only one copy of a multicast message will pass
  over any link in the network.

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Multicast Requirements

• Support for Multicast in the TCP/IP stack

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Multicast Requirements

• Support for Multicast in the TCP/IP stack
• Support for IGMP to communicate requests to join
  multicast groups and receive traffic

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Multicast Requirements

• Support for Multicast in the TCP/IP stack
• Support for IGMP to communicate requests to join
  multicast groups and receive traffic
• NICs which understand Multicast addresses

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Multicast Requirements

• Support for Multicast in the TCP/IP stack
• Support for IGMP to communicate requests to join
  multicast groups and receive traffic
• NICs which understand Multicast addresses
• IP Multicast-capable network routers

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Multicast Requirements

• Support for Multicast in the TCP/IP stack
• Support for IGMP to communicate requests to join
  multicast groups and receive traffic
• NICs which understand Multicast addresses
• IP Multicast-capable network routers
• Firewalls which permit IP Multicast traffic

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Multicast Requirements

• Support for Multicast in the TCP/IP stack
• Support for IGMP to communicate requests to join
  multicast groups and receive traffic
• NICs which understand Multicast addresses
• IP Multicast-capable network routers
• Firewalls which permit IP Multicast traffic
• IP Multicast application software

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Multicast-enabled components
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IP Multicast Addressing

• Class D 1110 as high-order four bits

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IP Multicast Addressing

• Class D 1110 as high-order four bits
• 256 million IP Multicast addresses

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IP Multicast Addressing

• Class D 1110 as high-order four bits
• 256 million IP Multicast addresses
• 224.0.0.0 to 239.255.255.255

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IP Multicast Addressing

• Class D 1110 as high-order four bits
• 256 million IP Multicast addresses
• 224.0.0.0 to 239.255.255.255
• Some permanent addresses, such as 224.0.0.1 (all
  hosts group on LAN).

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IP Multicast Addressing

• Class D 1110 as high-order four bits
• 256 million IP Multicast addresses
• 224.0.0.0 to 239.255.255.255
• Some permanent addresses, such as 224.0.0.1 (all
  hosts group on LAN).
• Some reserved, such as 224.0.0.0 to 224.0.0.255
  for routing protocols

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IP Multicast Addressing

• Class D 1110 as high-order four bits
• 256 million IP Multicast addresses
• 224.0.0.0 to 239.255.255.255
• Some permanent addresses, such as 224.0.0.1 (all
  hosts group on LAN).
• Some reserved, such as 224.0.0.0 to 224.0.0.255
  for routing protocols
• Automatic routing discovery
• Exchange link state information
• Rest are dynamically assigned as needed

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Receiving Multicast Datagrams

• Application requests membership in the group
  associated with a particular multicast

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Receiving Multicast Datagrams

• Application requests membership in the group
  associated with a particular multicast
• Request is communicated to the LAN router

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Receiving Multicast Datagrams

• Application requests membership in the group
  associated with a particular multicast
• Request is communicated to the LAN router
• Request may be forwarded to intermediate WAN
  routers between sender and receiver

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Receiving Multicast Datagrams

• Application requests membership in the group
  associated with a particular multicast
• Request is communicated to the LAN router
• Request may be forwarded to intermediate WAN
  routers between sender and receiver
• NIC starts filtering for multicast packets

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Receiving Multicast Datagrams

• Application requests membership in the group
  associated with a particular multicast
• Request is communicated to the LAN router
• Request may be forwarded to intermediate WAN
  routers between sender and receiver
• NIC starts filtering for multicast packets
• WAN routers deliver Multicast packets to LAN
  router, which maps to MAC address

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Binding

• IP unicast address is statically bound to a
  single NIC on a single IP network.

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Binding

• An IP unicast address is statically bound to a
  single NIC on a single IP network
• An IP host group address is dynamically bound to
  a set of NICs on a set of IP networks

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Binding

• An IP unicast address is statically bound to a
  single NIC on a single IP network
• An IP host group address is dynamically bound to
  a set of NICs on a set of IP networks
• An IP host group address is not bound to a set of
  IP unicast addresses

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Binding

• An IP unicast address is statically bound to a
  single NIC on a single IP network
• An IP host group address is dynamically bound to
  a set of NICs on a set of IP networks
• An IP host group address is not bound to a set of
  IP unicast addresses
• Multicast routers only need to know the groups
  for which there is one member on the subnetwork

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IGMP

• The Internet Group Management Protocol is used by
  multicast routers to learn the existence of host
  group members on their directly attached subnets.

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IGMP Messages

• To determine if any hosts on a local subnet
  belong to a multicast group, one multicast router
  per subnet periodically sends a hardware (data
  link layer) IGMP Host Membership Query to all IP
  end nodes on its LAN, asking them to report back
  on the host groups memberships of their
  processes. This query is sent to the all-hosts
  group (224.0.0.1).

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IGMP Messages (continued)
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IGMP Messages (continued)

• To avoid network congestion, each host delays
  its report by a random interval if it has not
  seen a report for the same group from another
  host. As a result, only one report is sent in
  response for each active group address per
  subnet, although many hosts may have memberships.

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IGMP Messages (continued)

• IGMP updates are used by multicasting routers to
  communicate host group memberships to neighboring
  routers, propagating group information through
  the internet.

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Routing

• Multicast routers interact with each other to
  exchange information about neighboring routers.
  To avoid duplication of effort, a single router
  is selected (via IGMP) as the Designated Router
  for each physical network.

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Spanning Trees

• For efficient transmission, Designated Routers
  construct a spanning tree that connects all
  members of an IP Multicast group.
• Minimal number of edges (transmission links) to
  guarantee delivery of source data packets to
  receivers

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Spanning Trees (continued)
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Dense-mode Multicasting

• The first approach is based on the assumption
  that the multicast group members are densely
  distributed throughout the network and bandwidth
  is plentiful,
• i.e., almost all hosts on the network belong to
  the group.
• So-called dense-mode multicast routing
  protocols rely on periodic flooding of the
  network with multicast traffic to set up and
  maintain the spanning tree.

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Sparse-mode Multicasting

• The second approach to multicast routing is based
  on the assumption that the multicast group
  members are sparsely distributed throughout the
  network and bandwidth is not necessarily widely
  available
• for example across many regions of the Internet
• Hence, sparse-mode'' multicast routing
  protocols must rely on more selective techniques
  to set up and maintain multicast trees.

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Conclusion

• IP Multicast enables many new types of
  applications and reduces network congestion and
  server loads.
• Advances are being made in areas such as reliable
  multicasting and real-time applications support.