The functions that provide the Standard Internet Multicast Service can be separated into host and network components. The interface between these is provided by IP multicast addressing and IGMP group membership functions, as well as standard IP packet transmission and reception. The Network functions are principally concerned with multicast routing, while host functions also include higher layer functions such as the adition of reliability facilities in a transport layer protocol. We cover each of these functions in that order, in the rest of this chapter.