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Chapter 15 Multicasting and Multicast Routing Protocols Objectives Upon completion you will be able to: • Differentiate between a unicast, multicast, and broadcast message • Know the many applications of multicasting • Understand multicast link state routing and MOSPF • Understand multicast link state routing and DVMRP • Understand the Core-Based Tree Protocol • Understand the Protocol Independent Multicast Protocols • Understand the MBONE concept TCP/IP Protocol Suite 1 15.1 UNICAST, MULTICAST, AND BROADCAST A message can be unicast, multicast, or broadcast. Let us clarify these terms as they relate to the Internet. The topics discussed in this section include: Unicasting Multicasting Broadcasting Multicasting versus Multiple Unicasting TCP/IP Protocol Suite 2 Figure 15.1 TCP/IP Protocol Suite Unicasting 3 Note: In unicasting, the router forwards the received packet through only one of its interfaces. TCP/IP Protocol Suite 4 Figure 15.2 TCP/IP Protocol Suite Multicasting 5 Note: In multicasting, the router may forward the received packet through several of its interfaces. TCP/IP Protocol Suite 6 Figure 15.3 TCP/IP Protocol Suite Multicasting versus multiple unicasting 7 Note: Emulation of multicasting through multiple unicasting is not efficient and may create long delays, particularly with a large group. TCP/IP Protocol Suite 8 15.2 MULTICAST APPLICATIONS Multicasting has many applications today such as access databases, information dissemination, teleconferencing, learning. to distributed and distance The topics discussed in this section include: Access to Distributed Databases Information Dissemination Dissemination of News Teleconferencing Distance Learning TCP/IP Protocol Suite 9 15.3 MULTICAST ROUTING In this section, we first all multicast protocols. protocols. discuss the idea of optimal routing, common in We then give an overview of multicast routing The topics discussed in this section include: Optimal Routing: Shortest Path Trees Routing Protocols TCP/IP Protocol Suite 10 Note: In unicast routing, each router in the domain has a table that defines a shortest path tree to possible destinations. TCP/IP Protocol Suite 11 Figure 15.4 TCP/IP Protocol Suite Shortest path tree in unicast routing 12 Note: In multicast routing, each involved router needs to construct a shortest path tree for each group. TCP/IP Protocol Suite 13 Note: In the source-based tree approach, each router needs to have one shortest path tree for each group. TCP/IP Protocol Suite 14 Figure 15.5 TCP/IP Protocol Suite Source-based tree approach 15 Figure 15.6 TCP/IP Protocol Suite Group-shared tree approach 16 Note: In the group-shared tree approach, only the core router, which has a shortest path tree for each group, is involved in multicasting. TCP/IP Protocol Suite 17 Figure 15.7 TCP/IP Protocol Suite Taxonomy of common multicast protocols 18 15.4 MULTICAST LINK STATE ROUTING: MOSPF In this section, we briefly discuss multicast link state routing and its implementation in the Internet, MOSPF. The topics discussed in this section include: Multicast Link State Routing MOSPF TCP/IP Protocol Suite 19 Note: Multicast link state routing uses the source-based tree approach. TCP/IP Protocol Suite 20 15.5 MULTICAST DISTANCE VECTOR: DVMRP In this section, we briefly discuss multicast distance vector routing and its implementation in the Internet, DVMRP. The topics discussed in this section include: Multicast Distance Vector Routing DVMRP TCP/IP Protocol Suite 21 Note: Flooding broadcasts packets, but creates loops in the systems. TCP/IP Protocol Suite 22 Note: RPF eliminates the loop in the flooding process. TCP/IP Protocol Suite 23 Figure 15.8 TCP/IP Protocol Suite RPF 24 Figure 15.9 TCP/IP Protocol Suite Problem with RPF 25 Figure 15.10 TCP/IP Protocol Suite RPF versus RPB 26 Note: RPB creates a shortest path broadcast tree from the source to each destination. It guarantees that each destination receives one and only one copy of the packet. TCP/IP Protocol Suite 27 Figure 15.11 TCP/IP Protocol Suite RPF, RPB, and RPM 28 Note: RPM adds pruning and grafting to RPB to create a multicast shortest path tree that supports dynamic membership changes. TCP/IP Protocol Suite 29 15.6 CBT The Core-Based Tree (CBT) protocol is a group-shared protocol that uses a core as the root of the tree. The autonomous system is divided into regions and a core (center router or rendezvous router) is chosen for each region. The topics discussed in this section include: Formation of the Tree Sending Multicast Packets Selecting the Rendezvous Router TCP/IP Protocol Suite 30 Figure 15.12 TCP/IP Protocol Suite Group-shared tree with rendezvous router 31 Figure 15.13 TCP/IP Protocol Suite Sending a multicast packet to the rendezvous router 32 Note: In CBT, the source sends the multicast packet (encapsulated in a unicast packet) to the core router. The core router decapsulates the packet and forwards it to all interested interfaces. TCP/IP Protocol Suite 33 15.7 PIM Protocol Independent Multicast (PIM) is the name given to two independent multicast routing protocols: Protocol Independent Multicast, Dense Mode (PIM-DM) and Protocol Independent Multicast, Sparse Mode (PIM-SM). The topics discussed in this section include: PIM-DM PIM-SM TCP/IP Protocol Suite 34 Note: PIM-DM is used in a dense multicast environment, such as a LAN. TCP/IP Protocol Suite 35 Note: PIM-DM uses RPF and pruning/grafting strategies to handle multicasting. However, it is independent from the underlying unicast protocol. TCP/IP Protocol Suite 36 Note: PIM-SM is used in a sparse multicast environment such as a WAN. TCP/IP Protocol Suite 37 Note: PIM-SM is similar to CBT but uses a simpler procedure. TCP/IP Protocol Suite 38 15.8 MBONE A multicast router may not find another multicast router in the neighborhood to forward the multicast packet. A solution for this problem is tunneling. We make a multicast backbone (MBONE) out of these isolated routers using the concept of tunneling. TCP/IP Protocol Suite 39 Figure 15.14 TCP/IP Protocol Suite Logical tunneling 40 Figure 15.15 TCP/IP Protocol Suite MBONE 41