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Multicast & Multimedia

Multicast & Multimedia. CS731 Wei Tsang Ooi. Overview. What is Multicast ? Group Management (IGMP) Routing Protocols (DVMRP, MOPSF, CBT, PIM-DM, PIM-SM) Adaptivity (DSG, RLM, ThinStreams, SCUBA, MEGA). What is Multicast ?. What is Multicast ?.

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Multicast & Multimedia

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  1. Multicast & Multimedia CS731 Wei Tsang Ooi

  2. Overview • What is Multicast ? • Group Management (IGMP) • Routing Protocols (DVMRP, MOPSF, CBT, PIM-DM, PIM-SM) • Adaptivity (DSG, RLM, ThinStreams, SCUBA, MEGA)

  3. What is Multicast ?

  4. What is Multicast ? • Sending a packet to multiple destination using a single address

  5. Unicast S A A B Router C B Router Router Router C

  6. Multicast S G A Router B Router Router Router C

  7. Multicast S A Router G B Router Router Router G C

  8. Multicast S G A Router B Router Router G Router C

  9. Questions • How does a router know if some host in its subnet is member of some group ? • How does a router know where to forward a packet ? • What if a packet is lost ? • What if the members have different network capacity ?

  10. Questions • How does a router know if some host in its subnet is member of some group ? • How does a router know where to forward a packet ? • What if a packet is lost ? • What if the members have different network capacity ?

  11. GroupManagement

  12. Group Management • Routers maintain “local host group membership table” • “which group has a member in my subnet ?”

  13. IGMP v1.0 • JOIN messageA : “I want to join group G.” • QUERY messageR : “Which group have you joined ?”

  14. IGMP v1.0 • MEMBERSHIP ReportA : “I am a member of group G”A : “I am a member of group H”B : “I am a member of group G”

  15. Avoiding Implosion • Select random delay t • After time t, if nobody belongs to the same group, send membership report. • Resend the report after some delay just to be safe.

  16. Leave • If nobody tell the router that they belongs to some group G after a few query messages, G will be removed from local host membership table. • Long leave latency (minutes)

  17. IGMP v2.0 • LEAVE message“I want to leave group G” • Group-Specific Query“Anybody else belongs to group G ?”

  18. IGMP v3.0 • Group-Source Inclusion“I want to listen to S from group G” • Group-Source Exclusion“I do not want to listen to S from group G”

  19. Questions • How does a router know if some host in its subnet is member of some group ? • How does a router know where to forward a packet ? • What if a packet is lost ? • What if the members have different network capacity ?

  20. Routing Protocols

  21. Routing Protocols • Generic Methods : • Form a tree to all routers with members • Deliver the packets along the tree

  22. Rounting Protocols • DVMRP • MOSPF • CBT • PIM

  23. Rounting Protocols • DVMRP • MOSPF • CBT • PIM

  24. From S to G Q R P T

  25. Is R on the shortest path to S ? Q R P T

  26. If no.. ignore the packet Q R P T

  27. If yes.. continue Where should I forward it to ? Q R P T

  28. Q R P T

  29. Q R P T

  30. Truncation • Router checks local host membership table. • Duplicate the packets only if there is a member in the subnet.

  31. Q ? R P T ?

  32. U S Q ? R P T ?

  33. Exchanging Routing Table • Routers periodically sends routing tables to their neighbours • If neighbour is going to ignore my packets, don’t need to send the packets to it.

  34. Pruning • Default : Always send to neighbouring routers, unless told otherwise. • Routers who received a “useless” packet send a prune message back.“Don’t send me packet addressed to G anymore !”

  35. Q R P T

  36. Q R P T PRUNE

  37. Q R P T

  38. Q R P T GRAFT

  39. Q R P T

  40. Q PRUNE R P T

  41. Problems of DVMRP • One tree for each pair (source, group) • Rebuilt tree periodically • So need to exchange routing tables periodically • First packet may visits more routers than it needs to

  42. Rounting Protocols • DVMRP • MOSPF • CBT • PIM

  43. MOSPF • Based on OSPF • For intra-gateway routing • Routers flood membership infomation to all other routers

  44. MOPSF • All routers must have the same topological/group members information. • Each of them compute a shortest path tree.

  45. Problems of MOSPF • Different minimal tree leads to wastage. P P Q R Q R

  46. Solution • Some convention to make sure all tree computed are the same

  47. Problems of MOSPF • One tree for each (source, group) pair • Computational intensive

  48. Solution • Compute the tree when received the first packet.

  49. Example of MOSPF P Q R U T

  50. P computes the tree P Q R U T

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