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##### Communication Networks

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**Communication Networks**Recitation 13 Multicast Routing Comnet 2006**The Problem**Traditional unicast model does not scale • Millions of clients • Server and network meltdown Comnet 2006**Solution: IP Multicast**• Source sends single stream • Routers split stream towards all clients • Guarantee only one copy in each link Comnet 2006**Multicast Routing Tree**Multicast Routing Protocol On tree relay router Router with directly attached group members IGMP Comnet 2006**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**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**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**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**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**Example Topology**g g s g Comnet 2006**Phase 1: Flooding**g g s g Comnet 2006**Phase 2: Pruning**g g prune (s,g) prune (s,g) s g Comnet 2006**Steady State**g g g s g Comnet 2006**Joining on New Receivers**g g g report (g) graft (s,g) graft (s,g) s g Comnet 2006**Steady State after Joining**g g g s g Comnet 2006**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**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**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**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**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**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**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