t routing, or unicast routing, a packet is routed to a sin- gle destination. In multicast routing, a single packet is routed to a set of destina- tions. Single LAN segments running a protocol such as Ethernet offer the capability for packet broadcast, where a packet is sent to every node on the net- work. Scaling this to a larger network with routers is a recipe for disaster, as SIP and the Internet broadcast traffic can quickly cause congestion. An alternative approach for this type of packet distribution is to use a packet reflector that receives packets and forwards copies to all destinations that are members of a broadcast group. This also can cause congestion in the form of a "packet storm" [16]. For a number of years, the Internet Multicast Backbone Network (MBONE), an overlay of the public Internet, has used multicast routing for high-bandwidth broadcast ses- sions. Participants who wish to join a multicast session send a request to join the session to their local MBONE router. That router will then begin to broadcast the multicast session on that LAN segment. Additional requests to join the ses- sion from others in the same LAN segment will result in no additional multicast packets being sent, since the packets are already being broadcast. If the router is not aware of any multicast participants on its segment, it will not forward any of the packets. Routing of multicast packets between routers uses special multicast routing protocols to ensure that packet traffic on the backbone is kept to a mini- mum. Multicast Internet addresses are reserved in the range 224.0.0.0 to 239. 255 255. 255. Multicast transport is always UDP, since the hand- shake and acknowledgments of TCP are not possible. Certain addresses have been defined for certain protocols and applications. The scope or extent of a multicast session can be limited using the time-to-live (TTL) field in the IP header. This field is decremented by each router that forwards the packet, which limits the number of hops the packet takes. SIP support for mu