Reminder Of The Communication Models#

client/server behind-NAT devices Point-to-point Hybrid Unicast Multicast

Multicast concepts#

Multicast clients must join multicast groups in order to receive multicast traffic.

  • IP host: an IP endpoint that is connected to a multicast router.
  • Multicast router: a router that supports multicast protocols such as PIM and IGMP.
  • upstream router
  • downstream router
  • multicast group
  • nulticast (source, group) pair
  • multicast group membership
  • interested node
  • Host Membership Query
  • Host Membership Query
  • Host Membership Report
  • random report delay timer
  • shared tree
  • source-rooted spanning tree
  • Multicast Distribution Tree
  • parent link
  • child link
  • active branches
  • multicast leaf router: is connected to leaf sub-networks
  • Leaf sub-network: some have group members, some do not.
  • DVMRP

Advantages of Multicast Communication#

Routers process one packet instead of many (in case of multiple unicast transmissions) Routers do not have to determine the destination address to send the packet to, in contrast to unicast transmission. Reduced usage of resources: router CPU and memory, network bandwidth.

Shortcomings of Multicast Communication#

Multicast communications are UDP based. Therefore:

  • No congestion control mechanism like TCP Slow Start,
  • Multicast packets arrive with no particular order, as opposed to TCP’s sequence numbering,
  • not everyone understands how to implement and troubleshoot multicast :-)

Multicast algorithms#

Spanning Tree Flooding Reverse Path Multicasting Reverse Path Broadcasting

Pruning#

Prune Message#

Prune Information#

Configuration#

Cisco IOS# 

(config)# ip multicast-routing
(config-if)# ip pim {sparse-mode OR dense-mode OR sparse-dense}