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Communication Networks
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  1. Communication Networks Recitation 13 Multicast Routing Comnet 2006

  2. The Problem Traditional unicast model does not scale • Millions of clients • Server and network meltdown Comnet 2006

  3. Solution: IP Multicast • Source sends single stream • Routers split stream towards all clients • Guarantee only one copy in each link Comnet 2006

  4. Multicast Routing Tree Multicast Routing Protocol On tree relay router Router with directly attached group members IGMP Comnet 2006

  5. Internet Group Management Protocol (IGMP) • Used by routers to learn about Multicast Group Memberships on their directly attached subnets • Implemented over IP • Designated Router • Each network has one Querier • All routers begin as Queriers • Router with the lowest IP address chosen Comnet 2006

  6. How IGMP Works one router is elected the “querier” querier periodically sends a Membership Query messageto the all-systems group (224.0.0.1), with TTL = 1 on receipt, hosts start random timers (between 0 and 10 seconds) for each multicast group to which they belong routers: Q hosts: Comnet 2006

  7. How IGMP Works (cont.) when a host’s timer for group G expires, it sends a Membership Report to group G, with TTL = 1 other members of G hear the report and stop their timers routers hear all reports, and time out non-responding groups Q G G G G Comnet 2006

  8. Shortest Path Tree (SPT) • Source Based Tree:Rooted at the source, composed of the shortest paths between the source and each of the receivers in the multicast group. • If the routing metric used is the latency between neighbors, the resulted tree will minimize delay over the multicast group. • Example: DVMRP. Comnet 2006

  9. Distance-Vector Multicast Routing Protocol (DMVRP) DVMRP consists of two major components: (1) a conventional distance-vector routing protocol (like RIP) (2) a protocol for determining how to forward multicast packets, based on the routing table and routing messages of (1) Comnet 2006

  10. Example Topology g g s g Comnet 2006

  11. Phase 1: Flooding g g s g Comnet 2006

  12. Phase 2: Pruning g g prune (s,g) prune (s,g) s g Comnet 2006

  13. Steady State g g g s g Comnet 2006

  14. Joining on New Receivers g g g report (g) graft (s,g) graft (s,g) s g Comnet 2006

  15. Steady State after Joining g g g s g Comnet 2006

  16. Steiner Minimal Tree (SMT) • Shared Tree: All sources use the same shared tree. • SMT is defined to be the minimal cost subgraph (tree) spanning a given subset of nodes in a graph • Approximate SMT: KMB Comnet 2006

  17. A Steiner Tree Example 10 • Which is the Steiner tree for green and red nodes? 10 10 3 2 3 5 4 1 2 1 Comnet 2006

  18. A Steiner Tree Example: Solution 10 • Shortest Path tree =/= Steiner Tree • 14 + 13 =/= 16 10 10 3 2 3 5 4 1 2 1 Comnet 2006

  19. KMB Algorithm • G=(V,E), terminals R • Step 1: Construct a complete directed distance graph of R: G1=(V1,E1,c1). • Step 2: Find the min spanning tree T1 of G1. • Step3: Construct a subgraph GS of G by replacing each edge in T1 by its corresponding shortest path in G. • Step 4: Find the min spanning tree TS of GS. • Step 5: Construct a Steiner tree TH from TS by deleting leaves that are not in R. Comnet 2006

  20. KMB Algorithm Cont. Due to [Kou, Markowsky and Berman 81’] Worst case time complexity O(|S||V|2). Cost no more than 2(1 - 1/l) *optimal cost where l = number of leaves in the steiner tree. Comnet 2006

  21. A D 4 A 4 1 4 4 4 4 H A D 4 1/2 I B C 1/2 G1 1 G 4 1 1 1 E 4 F B B C 2 2 T1 C D Gs KMB Example Destination Nodes A 1 10 H 1/2 I 1/2 G 1 1 1 1 B F E 8 2 2 C 9 D G Comnet 2006

  22. A A 1 1 H I 1/2 I 1/2 1 1 G 1 1 E 1 1 E F F B B 2 2 2 2 C D C D Ts TH KMB Example Cont. Comnet 2006