Many applications require data to be sent to multiple clients. This can be done with the use of IP Multicast. Class D internet addresses (range 224.*.*.* to 239.*.*.*) are reserved for multicasting. These addresses are not allowed to be assigned to a particular host, but instead, they act as broadcast channels that multiple hosts can tune into. To multicast, one must use the connectionless UDP instead of TCP, for TCP requires the existence of another host at the destination. Currently, most routers do not forward Multicast packets out of the the subnet it originated on. The main reason is because, if all routers were to forward a multicast packet to other connected subnets, then every multicast packet would be sent to every subnet on the Internet. This obviously would cause total congestion of the Internet. To allow multicast packet to travel outside of the subnet they origenated at, special multicast capable routers needed. These routers and special network channels called tunnels are used to establish a multicast network built on top of the Internet. This network spanning more than 50 countries is called the virtual Multicast Backbone On the InterNEt, also refered to as the MBONE. The multicast addresses reserved for the MBONE are 224.2.*.*, a small subset of the multicast addresses. Although this address space is small, if multicast packets were to go every where on the MBONE, then the part of the Internet on which the MBONE resided would also be congested. To avoid this, several techniques are implemented. All IP multicast packets have a TTL (Time To Live), a field in the IP header which routers decriment until it reaches zero when the packet is no longer forwarded to other networks. Also, multicast packets are only forwarded to subnets on the MBONE with hosts that would like to tune in to a specific multicast channel. If a host would like to listen to a particular multicast address/port, then it must send a join message as specified by IGMP, the Internet Group Management Protocol.
The list of most popular MBONE applications include vic and vat. vic and vat are video and audio conferencing tools respectively. A host on the MBONE can use these tools to multicast video and audio signals and listen in to other hosts multicast packets. These MBONE tools use RTP (Real-Time Protocol) to send the data.