1 Comnet 2010 Communication Networks Recitation 13 Multicast Routing

2 Comnet 2010 The Problem Traditional unicast model does not scale –Millions of clients –Server and network meltdown

3 Comnet 2010 Solution: IP Multicast Source sends single streamSource sends single stream Routers split stream towards all clientsRouters split stream towards all clients Guarantee only one copy in each linkGuarantee only one copy in each link

4 Comnet 2010 Multicast Routing Tree On tree relay router Router with directly attached group members IGMP Multicast Routing Protocol

5 Comnet 2010 Internet Group Management Protocol (IGMP) Used by routers to learn about Multicast Group Memberships on their directly attached subnetsUsed by routers to learn about Multicast Group Memberships on their directly attached subnets Implemented over IPImplemented over IP Designated RouterDesignated Router –Each network has one Querier –All routers begin as Queriers –Router with the lowest IP address chosen

6 Comnet 2010 How IGMP Works one router is elected the “querier” querier periodically sends a Membership Query message to the all-systems group ( ), with TTL = 1 on receipt, hosts start random timers (between 0 and 10 seconds) for each multicast group to which they belong Qrouters: hosts:

7 Comnet 2010 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 GGGG

8 Comnet 2010 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.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.If the routing metric used is the latency between neighbors, the resulted tree will minimize delay over the multicast group. Example: DVMRP.Example: DVMRP.

9 Comnet 2010 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)

10 Comnet 2010 Example Topology gg s g

11 Comnet 2010 Phase 1: Flooding gg s g

12 Comnet 2010 Phase 2: Pruning gg s prune (s,g) g

13 Comnet 2010 Steady State gg s g g

14 Comnet 2010 graft (s,g) Joining on New Receivers gg s g g report (g)

15 Comnet 2010 Steady State after Joining gg s g g

16 Comnet 2010 Steiner Minimal Tree (SMT) Shared Tree: All sources use the same shared tree.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 graphSMT is defined to be the minimal cost subgraph (tree) spanning a given subset of nodes in a graph Approximate SMT: KMBApproximate SMT: KMB

17 Comnet 2010 A Steiner Tree Example Which is the Steiner tree for green and red nodes?Which is the Steiner tree for green and red nodes?

18 Comnet 2010 A Steiner Tree Example: Solution Shortest Path tree =/= Steiner TreeShortest Path tree =/= Steiner Tree =/= =/=

19 Comnet 2010 G=(V,E), terminals RG=(V,E), terminals R Step 1: Construct a complete directed distance graph of R: G 1 =(V 1,E 1,c 1 ).Step 1: Construct a complete directed distance graph of R: G 1 =(V 1,E 1,c 1 ). Step 2: Find the min spanning tree T 1 of G 1.Step 2: Find the min spanning tree T 1 of G 1. Step3: Construct a subgraph G S of G by replacing each edge in T 1 by its corresponding shortest path in G.Step3: Construct a subgraph G S of G by replacing each edge in T 1 by its corresponding shortest path in G. Step 4: Find the min spanning tree T S of G S.Step 4: Find the min spanning tree T S of G S. Step 5: Construct a Steiner tree T H from T S by deleting leaves that are not in R.Step 5: Construct a Steiner tree T H from T S by deleting leaves that are not in R. KMB Algorithm

20 Comnet 2010 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. KMB Algorithm Cont.

21 Comnet 2010 KMB Example A B CD EF G H I /2 2 1 Destination Nodes G A C D B G1G1 A C D B T1T1 B CD E F G H I /2 2 1 A GsGs

22 Comnet 2010 KMB Example Cont. B CD E F G H I /2 2 A TsTs B CD E F I A THTH