MOSPF, currently under development, is a backward-compatible addition to OSPF that allows the forwarding of both IP multicasts and unicasts within an Autonomous System. MOSPF routers can be mixed with OSPF routers within a routing domain, and they will interoperate in the forwarding of unicasts. OSPF is a link-state or SPF-based protocol. By adding link state advertisements that pinpoint group membership, MOSPF routers can calculate the path of a multicast datagram as a tree rooted at the datagram source. Those branches that do not contain group members can then be discarded, eliminating unnecessary datagram forwarding hops.