he proportion of hosts receiving multi- cast data is high, the probability of having at least one member of the multicast group in any particular subnetwork is also high. Chapter 3 Dense mode protocols usually build a shortest-path tree per sender or group of senders. Thus, a different distribution tree is used depending on where the data comes from, which is why they're usually referred to as source-based trees. Figures 3-9, 3-10, and 3-11 show how three different dis- tribution trees are calculated for three different senders. Some common examples of dense mode protocols are Distance Vector Multicast Routing (DVMRP) [RFC 1075] and Protocol Independent Multi- cast-Dense Mode (PIM-DM). DVMRP is currently enjoying the most wide- spread use. Sparse Mode Multicast Routing Protocols Sparse mode protocols are better suited for networks where the proportion of hosts receiving multi- cast traffic is low. These protocols usually implement a rendezvous point and, based on it, build a shared tree that will be used by all sources. Figure 3-12 shows how a distribution tree for the rendezvous point is built. The tree is computed as if the rendezvous point were the sender and all the other group members were the receivers. While this tree is being computed, packets from the group's senders are encapsulated and routed towards the rendezvous point, which then delivers them to receivers by con- sulting the tree. Figure 3-9 153.88.251.19 Distribution tree for 131.160.1.112. 131.160.1.112 ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER 138.85.27.10 The Internet Multimedia Conferencing Architecture Figure 3-10 153.88.251.19 Distribution tree for 138.85.27.10. 131.160.1.112 ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER 138.85.27.10 Figure 3-11 153.88.251.19 Distribution tree for 153.88.251.19. 131.160.1.112 ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER 138.85.27.10 Once a tree for the rendezvous point is complete, all nodes use it. This means that packets are no longer encapsulated for transmission to the ren- dezvous point, b