t support pruning. Customers should not implement DVMRP for IP multicasting unless their routers support prune. DVMRP is used in the Internet's multicast backbone (MBONE) which has a following in the academic community. The MBONE's primary application is the transmission of conference proceedings although a variation of it (CU-SeeMe) also supports desktop conferencing. MOSPF was defined as an extension to the Open Shortest Path First (OSPF) unicast routing protocol. It will only work over networks that use OSPF. OSPF requires that every router in a network be aware of all of the network's available links. Each router calculates routes from itself to all possible destinations. MOSPF works by including multicast information in OSPF link state advertisements. An MOSPF router learns Video Communications: The Whole Picture which multicast groups are active on which LANs, and then builds a distribution tree for each source/group pair. The tree state is cached and recomputed every time that a link state change occurs (or when the cache times out). MOSPF works best when there are only a few simultaneous multicasts at any given time. It should be avoided by environments that have many, simultaneous multicasts or by environments plagued by link failures. PIM works with all existing unicast routing protocols, and is usually more popular from both a customer and a vendor perspective because it is simpler to implement and to manage. PIM supports two different types of multipoint traffic distribution patterns: dense and sparse. PIM-Dense mode (PIM-DM), a proper subset of PIM- Sparse Mode (PIM-SM) performs best when senders and receivers are in close proximity to one another. It is also best adapted to situations where there are few senders and many receivers. PIM handles high volumes of multicast traffic and it does well when the streams of multicast traffic are continual. PIM-DM uses Reverse Path Forwarding and resembles DVMRP. The most significant difference between DVMRP and dense-mode PIM is that PIM works with whatever un