414 CHAPTER 37: IP MULTICAST OVERVIEWFigure 100 Application positions of multicast-related protocolsMulticast group management protocolMulticast groups use Internet group management protocol (IGMP) as themanagement protocols. IGMP runs between the host and multicast router anddefines the membership establishment and maintenance mechanism betweenthem.Multicast routing protocolsA multicast routing protocol runs between multicast routers to create andmaintain multicast routes for correct and efficient forwarding of multicast packet.The multicast routing creates a loop-free data transmission path from one sourceto multiple receivers. The task of multicast routing protocols is to build up thedistribution tree architecture. A multicast router can use multiple methods to buildup a path for data transmission, that is, a distribution tree.As in unicast routing, the multicast routing can also be intra-domain orinter-domain. Intra-domain multicast routing is rather mature and protocolindependent multicast (PIM) is the most wildly used intra-domain protocol, whichcan work in collaboration with unicast routing protocols. The inter-domain routingfirst needs to solve how to transfer routing information between ASs. Since theASs may belong to different telecom carriers, the inter-domain routing informationmust contain carriers’ policies, in addition to distance information. Currently,inter-domain routing protocols include multicast source discovery protocol (MSDP)and MBGP multicast extension.RPF Mechanism for IPMulticast PacketsTo ensure that multicast packets reach a router along the shortest path, themulticast router must check the receiving interface of multicast packets dependingon the unicast routing table or a unicast routing table independently provided formulticast. This check mechanism is the basis for most multicast routing protocolsto perform multicast forwarding, and is known as Reverse Path Forwarding (RPF)check. A multicast router uses the source address of a received multicast packet toquery the unicast routing table or the independent multicast routing table todetermine that the receiving interface is on the shortest path from the receivingstation to the source. If a source tree is used, the source address is the address ofthe source host sending the multicast packet. If a shared tree is used, the sourceServerMulticast User DUser EUser CIGMPIGMPUser AUser BIGMPMBGP/MSDPPIMPIMAS1AS2
