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[...]... core only one tree per multicast group (used for multiple senders) • Sources send multicast data to the core, which then multicasts the data to the tree • Multicast backbone of the Internet • Not all routers support multicast routing protocols and IGMP • Connecting multicast- capable routers using (virtual) IP tunnels • Was a long-running experimental approach to enabling multicast between sites through... forwarding, i indicates order link added by algorithm 25 26 13 Core-Based Trees MBone • For many-to-many multicast • Protocols: CBT, PIM-SM, PIM-DM (Protocol Independent Multicast, sparse/dense mode) • Purpose: to reduce the amount of routing info stored at routers when a multicast group has a large number of members and multiple senders • A multicast group requires a core (rendez-vous point) • Receivers... then specific protocols adopting these approaches 23 24 12 Shortest Path Tree MOSPF • Multicast cast forwarding tree: tree of shortest path routes from source to all receivers – Dijkstra’s algorithm S: source LEGEND R1 1 2 R4 R2 3 R3 • Extends OSPF for multicasting • Every router has the complete topology of its autonomous system • A receiver joins a multicast group G by exchanging IGMP messages with... end-router R • The end-router R broadcasts its group membership to the whole network in the form (G, R) • Every router in the network maintains a group membership table with each entry being a tuple [S, G, ] • A sender simply sends data packets as they are available • Each router uses the network topology, the group membership table, and the multicast group ID in the data packets to compute the... pruned: – multicast data again flows down unpruned branches – downstream routers: reprune or else continue to receive data • Routers can quickly re-graft to tree following an IGMP join at a leaf router by sending a “graft” message upstream • Deployment: – commonly implemented in commercial routers – Mbone routing done using DVMRP • Works well in small autonomous domains • Distance Vector Multicast. .. out • Between flooding rounds, a leaf router can re-join a group by sending a graft message upstream • Intermediate routers propagates the graft message upstream until the path is re-connected 21 22 11 Multicast Routing Approaches Steiner Trees • Steiner Tree: minimum cost tree connecting all routers with attached group members • Problem is NP-complete • Excellent heuristics exist • Not used in practice:... multicast- capable routers using (virtual) IP tunnels • Was a long-running experimental approach to enabling multicast between sites through the use of tunnels • No longer operational 27 28 14 References • Multicasting on the Internet and Its Applications, Sanjoy Paul, Kluwer Academic Publishers, 1998, chapters 2, 4, 5 • Computer Networking: A Top-Down Approach, 5th edition, Kurose and Ross 29 15 . 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