Multicast Routing

1
Multicast Routing
2
Multicasting

• Sending message to multicast address
• Multicast address refers to a group of hosts
• Multimedia
• Teleconferencing
• Databases
• Distributed computation
• Real-time workgroup

3
Multicasting within LAN

• MAC level multicast addresses
• IEEE 802 uses highest order bit 1
• All stations that recognise the multicast address
  accept the packet
• Works because of broadcast nature of LAN
• Packet only sent once
• Much harder on internet

4
Multicast one sender to many receivers

• Multicast act of sending datagram to multiple
  receivers with single transmit operation
• analogy one teacher to many students
• Question how to achieve multicast

5
Multicast one sender to many receivers

• Multicast act of sending datagram to multiple
  receivers with single transmit operation
• analogy one teacher to many students
• Question how to achieve multicast

• Application-layer multicast
• end systems involved in multicast copy and
  forward unicast datagrams among themselves

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Internet Multicast Service Model
128.59.16.12
128.119.40.186
multicast group 226.17.30.197
128.34.108.63
128.34.108.60

• multicast group concept use of indirection
• hosts addresses IP datagram to multicast group
• routers forward multicast datagrams to hosts that
  have joined that multicast group

7
Multicast groups

• class D Internet addresses reserved for
  multicast
• host group semantics
• anyone can join (receive) multicast group
• anyone can send to multicast group
• no network-layer identification to hosts of
  members
• needed infrastructure to deliver mcast-addressed
  datagrams to all hosts that have joined that
  multicast group

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Requirements for Multicasting (1)

• Router must forward two or more copies of
  incoming packet
• Addressing
• IPv4 uses class D
• Start 1110 plus 28 bit group id
• IPv6 uses 8 bit prefix of all 1s, 4 bit flags
  field, 4 bit scope field 112 bit group id
• Node must translate between multicast address and
  list of networks containing members of group
• Router must translate between IP multicast
  address and subnet multicast address to deliver
  to destination network

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Requirements for Multicasting (2)

• Multicast addresses may be permanent or dynamic
• Individual hosts may join or leave dynamically
• Need mechanism to inform routers
• Routers exchange information on which subnets
  contain members of groups
• Routers exchange information to calculate
  shortest path to each network
• Need routing protocol and algorithm
• Routes determined based on source and destination
  addresses
• Avoids unnecessary duplication of packets

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Joining a mcast group two-step process

• local host informs local mcast router of desire
  to join group IGMP (Internet Group Management
  Protocol)
• wide area local router interacts with other
  routers to receive mcast datagram flow
• many protocols (e.g., DVMRP, MOSPF, PIM)

IGMP
IGMP
wide-area multicast routing
IGMP
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IGMP Internet Group Management Protocol

• IGMP is a group management protocol.
• It helps a multicast router create and update a
  list of members related to each router interface.
• Position of IGMP in the network layer

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IGMP Internet Group Management Protocol

• host sends IGMP report when application joins
  mcast group to make itself known as member of
  group to other hosts and routers
• To join, send IGMP membership report message
• IP_ADD_MEMBERSHIP socket option
• host need not explicitly unjoin group when
  leaving.
• Note In IGMP, a membership report is sent twice,
  one after the other.

report
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IGMP Internet Group Management Protocol

• router periodically issues IGMP query
• To all-hosts multicast address
• Hosts respond with report message for each group
  to which it belongs
• Only one host in group needs to respond to keep
  group alive
• Host keeps timer and responds if no other reply
  heard in time

report
query
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IGMP

• IGMP v2 additions include
• group-specific Query
• Leave Group msg
• last host replying to Query can send explicit
  Leave Group msg
• router performs group-specific query to see if
  any hosts left in group
• RFC 2236
• IGMP v3 adds support for "source filtering",
  that is, the ability for a system to report
  interest in receiving packets only from
  specific source addresses, or from all but
  specific source addresses, sent to a particular
  multicast address. RFC3376

• IGMP version 1
• router Host Membership Query msg broadcast on
  LAN to all hosts
• host Host Membership Report msg to indicate
  group membership
• randomized delay before responding
• implicit leave via no reply to Query
• RFC 1112

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IGMP Operation Example
Imagine there are three hosts in a network as
shown in the next slide.

• A query message was received at time 0
• the random delay time (in tenths of seconds) for
  each group is shown next to the group address.
• Show the sequence of report messages.

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IGMP Operation Example
17
IGMP Operation Example
The events occur in this sequence Time 12 The
timer for 228.42.0.0 in host A expires and a
membership report is sent, which is received by
the router and every host including host B which
cancels its timer for 228.42.0.0. Time 30 The
timer for 225.14.0.0 in host A expires and a
membership report is sent, which is received by
the router and every host including host C which
cancels its timer for 225.14.0.0.
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IGMP Operation Example
Time 50 The timer for 251.70.0.0 in host B
expires and a membership report is sent, which is
received by the router and every host. Time 70
The timer for 230.43.0.0 in host C expires and a
membership report is sent, which is received by
the router and every host including host A which
cancels its timer for 230.43.0.0. Note that if
each host had sent a report for every group in
its list, there would have been seven reports
with this strategy only four reports are sent.
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Group Membership with IPv6

• Function incorporated in ICMPv6
• Includes all ICMPv4 plus IGMP
• Includes group membership query and report
• Addition of new group membership termination
  message

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Multicast Routing Problem Statement

• Goal find a tree (or trees) connecting routers
  having local mcast group members
• tree not all paths between routers used
• source-based different tree from each sender to
  rcvrs
• shared-tree same tree used by all group members

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Approaches for building mcast trees

• Approaches
• source-based tree one tree per source
• shortest path trees
• reverse path forwarding
• group-shared tree group uses one tree
• minimal spanning (Steiner)
• center-based trees

we first look at basic approaches, then specific
protocols adopting these approaches
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Building multicast trees (cntd.)

• Source Based Trees
• Notation (S, G)
• specific sender
• In a source-based tree approach, the combination
  of source and group determines the tree.
• Uses more memory (O(SG)), but can give optimal
  paths and delay.
• Group Shared Trees
• Notation (, G)
• All senders
• In the group-shared tree approach, the group
  determines the tree.
• Uses less memory (O(G)) but suboptimal paths and
  delays
• Data-driven
• Build when data packets are sent
• Demand-driven
• Build when members join

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Shortest Path Tree

• mcast forwarding tree tree of shortest path
  routes from source to all receivers
• Dijkstras algorithm

S source
LEGEND
R1
R4
router with attached group member
R2
router with no attached group member
R5
link used for forwarding, i indicates order
link added by algorithm
R3
R7
R6
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Center-based trees an example
Suppose R6 chosen as center
LEGEND
R1
router with attached group member
R4
3
router with no attached group member
R2
2
1
R5
path order in which join messages generated
R3
1
R7
R6
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Multicast routing protocols