ut are routed towards receivers directly through the tree. We can see in Figure 3-13 that the shared distribution tree used by nodes Chapter 3 Figure 3-12 153.88.251.19 Distribution tree for the rendezvous 131.160.1.112 point. ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER 138.85.27.10 with IP addresses 138.85.27.10 and 153.88.251.19 differs from the one cal- culated for them specifically in Figure 3-13. Examples of sparse mode protocols are Protocol Independent Multicast- Sparse Mode (PIM-SM) [RFC 2362] and Core-Based Trees (CBTs) [RFC 2189]. Apart from the protocol used to build the distribution tree, a host wishing to receive multicast data needs to become a member of a particular multi- cast group - a set of hosts that subscribe to the same multicast address. Internet Group Management Protocol (IGMP) [RFC 2236] [draft-ietf-idmr- igmp-v3] is used for this purpose (see Figure 3-14). Hosts send requests for joining or leaving a particular group. With this membership information in hand, a multicast router handling a certain subnetwork knows whether it needs to receive multicast data or not. If its subnetwork has no members, it does not need to receive datagrams addressed to the multicast address of the group and can accordingly remove itself from the distribution tree using a multicast routing protocol. Later, if a host in its subnetwork wishes to become a member of the multicast group, that router will be added to the distribution tree again. The Internet Multimedia Conferencing Architecture Figure 3-13 153.88.251.19 Shared tree for all the nodes. 131.160.1.112 ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER Rendezvous point 138.85.27.10 153.8.251.19 Figure 3-14 Use of IGMP. 131.160.1.112 Multicast Routing Protocols 138.85.27.10 Chapter 3 The Mbone The portion of the Internet that supports multicast is called the Mbone. The Mbone is not a separate network from the Internet. Mbone routers are also part of the Internet and are enabled for both unicast and multicast traffic. When an island of multicast routers 