MOSPF, a multicast routing protocol, is an enhancement of the unicast routing protocol OSPF [Mo94a]. OSPF is a link-state routing protocol in which the routers advertize the state of their directly attached links and based on these advertisements, each router builds up a link-state database. The OSPF link-state database provides a complete picture of the topology of an Autonomous System (AS). In order to support multicast routing, a new type of link-state advertisement, calledgroup-membership-LSA, has been added to OSPF. These advertisements help to pin-point the location of all multicast group members in the database. The path of a multicast datagram can then be calculated by building a shortest-path tree rooted at the datagram’s source. Note that each router in the domain has the complete description of the topology and the membership information and each one of them uses exactly the same algorithm to compute the shortest-path tree rooted at the same node. Thus every router ends up computing the same tree and creating the corresponding forwarding entries for each group. The shortest path treesare built on demand (that is, when the first datagram arrives) and the results of this computation are cached for use by subsequent datagrams having the same source and destination.