EE689 Lecture 12

1
EE689 Lecture 12

• Review of last lecture
• Multicast basics

2
Multicast -motivation

• Point-to-point delivery not efficient for
  events/transmissions of general interest
• Examples
• News multicast
• IETF sessions/rock concerts
• Many receivers may share the same path
• Point-to-point delivery too expensive

3
Multicast motivation
4
Multicast issues

• In point-to-point delivery, receiver address is
  known gt routing is straightforward
• In Multicast, many receivers
• How to transmit data to all the receivers?
• How to identify the group of receivers?
• At the sender?
• In the network?

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Multicast issues

• Identify multicast by a group/multicast address
• The membership changes as the receivers
  join/leave the group
• Routers/Network need to recognize the multicast
  address
• Receivers need to receive on their own IP address
  and on the multicast address

6
Multicast routing

• For point-to-point delivery, a router looks up
  routing table and knows how to forward
• For multicast, many receivers
• may have to forward packets on multiple outgoing
  links
• too hard to keep track of individual receivers at
  each router for each multicast group
• remember just the links on which to be forwarded
  - change as receivers join/leave

7
Multicast addressing

• A multicast sender uses the group address as the
  receivers address when sending packets
• Network/routers keep track of actual receiving
  subnets/paths for this group address (not the
  actual receivers)
• Receivers can reply to senders address or to
  group address

8
Multicast addressing

• Part of IP address space reserved for multicast
• Multicast routers recognize multicast addresses
• Need to get a multicast address for a
  transmission before multicast can start
• Protocols exist for obtaining multicast addresses
  and finding out a multicast address

9
Class D addresses

• High order 4 bits 1110, followed by a 28-bit
  multicast group ID. 224.0.0.0 - 239.255.255.255
• 224.0.0.1 - all systems on this subnet
• 224.0.0.2 - all routers on this subnet
• 224.0.0.4 - all DVMRP routers
• 224.0.0.5 - all OSPF routers

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Multicasting

• The end routers use physical layer multicasting
  to multicast packets to multiple receivers on the
  same subnet
• IP-multicast group ID copied into the MAC-layer
  multicast address
• All receivers of this group listen to this
  multicast address to receive packets

11
IGMP

• Internet Group Membership Protocol
• Used to join a multicast group and to check on
  the current status of receivers on a subnet
• IGMP -join message propagated up the routers
  until the multicast tree reached.
• Routers periodically poll hosts on subnets to see
  if any active receivers remain

12
IGMP

• If no active receivers remain, routers propagate
  leave messages upstream to reduce unnecessary
  traffic
• Frequent polling can increase overhead
• Separate protocols for finding group membership
  address - sd

13
IGMP version 3

• Allows sender-specific reports when multiple
  senders are in a multicast group
• Can exclude certain senders and allow certain
  senders - allows better bandwidth management in
  multi-sender multicasts.

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Mbone

• Multicast Backbone
• Consists of all the multicast-enabled routers
• If two multicast routers are not directly
  connected, uses tunneling over non-multicast
  routers
• Allows gradual deployment

15
Multicast routing

• Need to recognize multicast addresses
• The routing tables change as the receivers
  join/leave a multicast group
• Every router keeps track of downstream links on
  which to forward a packet
• Routing table and multicast address expire at
  the end of session

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Multicast routing

• Many possible approaches
• Flooding
• send on all links to reach the receivers
• not efficient
• Spanning tree
• efficient
• could concentrate traffic on a few links

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Routing

• Spanning trees rooted at the sender
• Sender sends out a broadcast over the entire
  network - all routers get at least one packet
• When receivers want to join, routers employ
  Reverse Path Multicasting
• Use Pruning to limit the multicast transmission.

18
Routing

• DVMRP - Distance Vector Multicast
• MOSPF - Multicast Extensions to OSPF
• PIM - Protocol Independent Multicast

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Routing

• Spanning trees rooted at the sender
• When a receiver wants to join a group, may have
  to broadcast on all upstream links to find a path
  to the sender
• could cause a lot of overhead in sparse groups
• need better solutions

20
Sparse Mode routing

• Use a Core-based tree (CBT)
• Use a rendezvous point or a core router
• Direct all joins to RP which knows how to reach
  the sender
• can avoid broadcasting multicast group
  information to all routers in the network
• can result in non-optimal paths
• would need to modify multicast tree over time