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EE689 Lecture 13

EE689 Lecture 13. Review of Last Lecture Reliable Multicast. 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 receiver’s status ACK Implosion.

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EE689 Lecture 13

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  1. EE689 Lecture 13 • Review of Last Lecture • Reliable Multicast

  2. 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 receiver’s status • ACK Implosion

  3. Receiver-driven Multicast • Sender keeps no information of receivers’ status • Receivers send NAKs to reduce ACK implosion problem • Several receivers may not get a packet on a loss - still possible to get many replies from receivers

  4. Receiver-driven Multicast • 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 • Multicast NAKs to sender • Others missing packets need not send NAKs • Could cause burden on receivers if only one receiver doesn’t get the packet

  5. Receiver-driven Multicast • Multicasting 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

  6. Receiver-driven Multicast • More scalable than sender-driven • Loss recovery times may be larger • Receiver may not know if last packet is lost • Realizes packet loss after receiving an out-of-sequence packet • Has to send a NAK and then reply from sender • May be faster if sender times out if ACK is not received in time

  7. Loss Recovery • Sender remulticasts packet to everyone • Possible to organize packet retransmissions into another multicast group • Receivers listen to the loss-multicast groups only on packet losses • could be more scalable

  8. Hierarchical Multicast • Organize multicast into a number of groups • One Designated Receiver takes responsibility for reliability • On packet loss, NAK propagated to DR • If DR has data, retransmits or remulticasts with limited scope to the group • If DR doesn’t have data, sends NAK to sender

  9. Hierarchical Multicast • More scalable than other multicast protocols • Need mechanisms to find out DR • Need mechanisms to delegate DR function to another node as primary DR node leaves multicast • Specially useful when multicast over wide geographic boundaries, keep one DR in each country for example

  10. Hierarchical Multicast • RMTP: Reliable Multicast Transport Protocol - Bell Labs • DR nodes may need more power than other receivers • Extra overhead in protocol • Quicker recovery times when losses are local

  11. Token-ring Multicast • Pass a token around a local group • Have token => DR for that packet/segment • Distribute responsibility over time • Need protocol for passing token around • Need protocol for associating a receiver with data segments • Again more scalable than flat multicast

  12. Heterogeneous Receivers • Not all the receivers may have same Bandwidth connectivity • Normally would result in sending data at the lowest quality that everyone can receive • At higher qualities • Too many packets dropped • Too much load on sender on retransmissions • everyone suffers delays or loss in quality

  13. Heterogeneous Receivers • Send video in multiple layers • Base layer and enhancement layers • Base layer provides the least quality • For example, at 28.8 kbs • Enhancement layers can be added if higher BW connectivity available • Higher quality video for some

  14. Heterogeneous Receivers • Layered video - use multiple multicast groups • Subscribe to all of them or some of them based on BW connectivity • This strategy works well with dynamic network conditions • congestion ( or BW availability) changes over time

  15. Network dynamics -multicast • When BW is plentiful (low loss rate), subscribe to all layers • As loss rate increases, subscribe to few layers • Sender transmits at the maximum level subscribed by the receiver group • Different layers will have different trees of distribution

  16. Multicast - Compression • Multi layer video coding popular • Some applications adjust rates on the layers dynamically • RLM - UC Berkeley, Café Mocha - TAMU • Vxtreme, RealVideo • Number of other tools

  17. Multicast Summary • Reliability poses interesting challenges • Receiver-driven multicast more scalable • Hierarchical multicast more scalable • Receiver Heterogenity forces video multicast to be layered • allows flexible QOS /distribution of video

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