Information Technology Reference
In-Depth Information
Figure 1.8
IP multicast.
Some well-known multicast routing protocols include the Distance Vector
Multicast Routing Protocol (DVMRP), the Protocol-Independent Multicast
in Sparse Mode (PIM-SM), Protocol-Independent Multicast in Dense Mode
(PIM-DM), the Core-Base Tree, and the Multicast Open Shortest Path First
Protocol (MOSPF). The DVMRP constructs a source-based tree using Reverse
Path Forwarding (RPF). The PIM-DM employs a simpler mechanism similar to
DVMRP to build a source-based tree. The MOSPF utilizes a protocol extend-
ing from the Open Shortest Path First (OSPF) Protocol. The CBT and PIM-SM
organizes their multicast trees using a core or Rendezvous Point (RP).
Even though the IP multicast provides bandwidth efficiency, it has several
disadvantages [10]. First, as mentioned earlier, IP multicast is infrastructure
dependent. Without the support of the routing devices, an IP multicast cannot
work. Second, because the edge router needs to maintain the membership infor-
mation of the end hosts, it lacks scalability as the number of multicast groups
increase. Third, it is not easy to implement access control or pricing policy over
IP multicasts. Because of some of these disadvantages, even if most routers have
the ability to support multicast, ISPs will generally not activate this function.
1.2.2.2 Application Layer Multicast
Figure 1.9 shows the conceptual diagram of ALM. In ALM, the server deliv-
ers the video content to certain users and the users that receive the content
