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Distance Vector Multicast Routing Protocol (DVMRP) DVMRP is the oldest multicast routing protocol and uses a technique known as reverse path forwarding. When a router receives a multicast packet (i.e., one with a Class D destination address), it floods the packet out of all paths except the one that leads back to the packet's source, as shown in Exhibit 2-1-6. This allows the packet to reach all subnetworks, possibly multiple times. If a router is attached to a set of subnetworks that do not want to receive packets destined to a particular multicast group, the router can send a prune message back up the distribution tree to stop subsequent multicast packets from being forwarded to destinations where there are no members.
DVMRP periodically refloods to reach any new hosts that want to receive a particular group. There is a direct relationship between the time it takes for a new receiver to get the data stream and the frequency of flooding. DVMRP implements its own unicast routing protocol to determine which interface leads back to the source of the packets for a particular group. This unicast routing protocol is similar to RIP and is based purely on hop count. As a result, the path that the multicast traffic follows may not be the same as the path the unicast traffic follows. DVMRP is an inefficient multicast routing protocol because of scaling problems. The periodic flooding with broadcast traffic generates significant overhead in the network, and early versions of the protocol did not implement pruning. Nevertheless, DVMRP has been used to build the Mbone and has been implemented by some router companies. Bay Networks, Inc. (Santa Clara CA) has chosen DVMRP as its first offering to support multicast routing, and 3COM Corp. (Santa Clara CA) is planning to implement it in the next few months. Multicast Open Shortest Path First (MOSPF) MOSFP is an extension of the OSPF unicast routing protocol. OSPF is a link state routing protocol, which means that each router in a network understands all of the available links in the network. Each OSPF router calculates the routes from itself to all possible destinations. MOSPF includes multicast information in OSPF link state advertisements. MOSPF routers learn which multicast groups are active on which subnetworks. MOSPF builds a distribution tree for each source/group pair and computes a tree for active sources sending to the group. The tree states are stored, and trees must be recomputed when a link state change occurs or when the timer for the store of the link expires. The main disadvantage of MOSPF is that it works only in networks supporting OSPF. MOSPF, authored by Proteon, Inc. (Westborough MA) has been implemented in routers available from Proteon and Xyplex, Inc. (Littleton MA). Protocol Independent Multicast (PIM) PIM works with all existing unicast routing protocols and supports two types of traffic distribution topologies: dense and sparse. Dense mode is most useful when:
Dense mode PIM uses reverse path forwarding and is similar to DVMRP. Sparse mode PIM is most useful in network topologies scattered over a large geographic area when:
Sparse mode PIM is designed for environments where there are many data streams destined for group members, as shown in Exhibit 2-1-7. Each stream goes to a relatively small number of group members. For this topology, reverse path forwarding techniques waste bandwidth.
Sparse mode PIM defines a rendezvous point (RP). Receiving members of the group join to a particular RP to create a shared distribution tree. There can be many RPs in a single group. Sources must register with all RPs in a group. Once the data stream begins to flow from sender to RP to receiver, the routers optimizes the path automatically to remove any unnecessary hops. Sparse mode PIM assumes that no hosts want the multicast traffic unless they specifically request it. PIM simultaneously supports dense and sparse mode for different groups. PIM was authored by Cisco Systems, Inc. (San Jose CA) and researchers from the University of Southern California, Lawrence B. Livermore Laboratory, and Xerox Parc. Work is continuing in the Internet community to improve multicast routing protocols. APPLICATIONS Dynamic Groups Groups defined by Class D addresses can be created and torn down in seconds. (Groups that are easily created and torn down are referred to as dynamic.) Changes to the group can also be performed in seconds, which is useful in many applications. Videoconferencing Videoconferencing is a temporary group setup that may exist for the duration of a meeting of any length. It is essentially an electronic meeting. Dataconferencing Dataconferencing is the same as videoconferencing without the video. Dataconferencing often involves a whiteboard, such as the program used by the academic community in the Mbone. Real-Time Distribution of Multimedia Information There are several new services that provide real-time business news to the desktop. Some of the information provided includes audio, video, and data. Subscribers to these services are constantly entering and leaving the group, therefore the ability to change the size and composition of the group dynamically is very important for this application.
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