CIS679: Multicast and Multimedia more

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CIS679 Multicast and Multimedia (more)

• Review of Last Lecture
• More about Multicast

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Review of Last Lecture

• Multicast basics
• Motivation and Issues
• Addressing
• Routing

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Reliable Multicast

• In unicast, receiver ACKs give feedback ---Sender
  takes responsibility in transmitting data
• In Multicast, many receivers -- too difficult for
  sender to keep track of every receivers status
• ACK Implosion

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Receiver-driven Multicast

• Sender based schemes dont scale well as number
  of receivers increase
• Receiver based schemes scale better
• Receivers can decide the level of reliability
  needed as well as the level of quality desired
  etc.

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Send NAKs

• Sender keeps no information of receivers status
• Receivers send NAKs to reduce ACK implosion
  problem
• How to send NAKs?
• Unicast NAKs to sender
• Multicast NAKs

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Unicast NAKs to sender

• Reduces overhead when packet losses are isolated
  and rare
• Packet loss early in the tree will result in too
  many NAKs

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Multicast NAKs

• Others missing packets need not send NAKs
• if every receiver, sends a NAK immediately after
  getting an out-of-sequence packet, too many NAKS
  at once!
• Wait for a random time, send a NAK
• If some one else sends a NAK, suppress your NAK
• Getting random timers tricky business
• Could cause burden on receivers if only one
  receiver doesnt get the packet

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Hierarchical Multicast

• Organize multicast into a number of groups
• One Designated Receiver (DR) takes responsibility
  for reliability
• On packet loss, NAK propagated to DR
• If DR has data, retransmits or re-multicasts with
  limited scope to the group
• If DR doesnt have data, sends NAK to sender

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Hierarchical Multicast

• More scalable than other multicast protocols
• Specially useful when multicast over wide
  geographic boundaries, keep one DR in each
  country for example
• DR nodes may need more power than other receivers
• Need mechanisms to find out DR
• Need mechanisms to delegate DR function to
  another node as primary DR node leaves multicast
• RMTP Reliable Multicast Transport Protocol -
  Bell Labs

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Congestion control

• Layered multicast
• Arrange layers in an exponentially increasing
  data rates
• TCP-friendly Multicast
• In steady state, packet drop gt congestion, drop
  a layer
• If layers are doubling in data rates, dropped
  layer reducing multicast rate by half gt TCP
  friendly

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QoS-Sensitive Multicast

• The key issue is to construct a multicast tree
  with QoS constraints
• Goal is to build a tree of paths to destinations
  such that sum of link costs (e.g. consumed
  bandwidth) is minimum and QoS constraints (e.g.
  delay) are satisfied
• Exact solutions to such multi-constrained
  optimization problems are prohibitively expensive
• Need heuristics that provide fast solutions of
  high quality

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An Example for Constructing A Tree

• Application QoS requirements end-to-end delay
• 13, jitter 7

• example 1

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10
10

• example 2

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6
6
10
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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

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Video Conferencing

• Vic is a video conferencing application developed
  by UC Berkeley
• It is a real-time, multimedia application for
  video conferencing over the internet.
• It is based on Real-time Transport Protocol
  (RTP).
• To run vis, the system must support multicast,
  ideally, support Mbone.
• An Intra-H.261 video encoder is combined.
• Further reading Steven McCanne and Van
  Jacobson, A Flexible Framework for Packet
  Video, ACM Multimedia 95

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Conclusion

• Reliable multicast
• Congestion control
• QoS Multicast
• Mbone
• Videoconferencing