Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 264
Multicast Algorithms

A lot of people do not even realize that they have already worked with multicasting. If you have worked with the Spanning Tree algorithm for bridging or the Open Shortest Path First (OSPF) protocol for IP routing updates, you have already worked with a multicast algorithm.

IP multicasting for subnet routing uses the following protocols:

•  Distance Vector Multicast Routing Protocol (DVMRP)

•  Multicast Open Shortest Path First (MOSPF)

•  Protocol Independent Multicast (PIM)

•  Sparse mode

•  Dense mode

There are essentially three forwarding algorithms that can be used with IP multicasting:

•  Flooding

•  Spanning Tree

•  Simple Spanning Tree

•  Reverse Path Broadcasting

•  Reverse Path Multicasting (most widely implemented)

•  Core-Based Trees (Used in sparse environments [environments that do not have a densely populated environment of hosts], CBT is a type of spanning tree algorithm but different enough to merit its own category).

The purpose of all the algorithms is to build a multicast tree for the forwarding of multicast datagrams. Some algorithms (CBT and sometimes PIM-SM) build only one tree that all members of the group share, even if the tree does not supply the most efficient (shortest path) route between all members of the group. Other protocols build shortest path trees that allow for the shortest path for all members in the group (DVMRP, OSPF). There can and will be multiple multicast trees built for each source/group on a network. These trees are built dynamically when the first multicast datagram arrives from the source (with the exception of MOSPF).

Multicast datagrams do not necessarily follow the unicast datagram’s path. The multicast tree that is built is a dynamic logical tree that a router will build to forward multicast datagrams to its receivers.