IP multicast: principles and protocols

In IP networks, packets are generally routed from a single source to a single receiver via routers. This type of transmission, known as IP unicast mode, has proven its effectiveness for point-to-point transmissions.

However, some applications (broadcasting audio/video content, etc.) require IP packets to be delivered to multiple destinations. One solution is for these applications to send a copy of each packet to each recipient in unicast mode. However, when the number of receivers is large, this solution reaches its limits, as the source must maintain a complete list of receivers, and the same data is transported several times over the same links. This approach requires additional bandwidth resources on the network, and therefore generates higher infrastructure costs (all the more inefficient when transport networks, such as Ethernet, natively support point-to-multipoint transmissions). In such cases, it is far more efficient to transmit data to multiple destinations using the IP multicast transmission mode.

Unlike IP unicast mode, IP multicast (specifically defined to support IP broadcast services) provides an efficient method for transporting multipoint-to-multipoint communications. IP multicast is a selective broadcast mode enabling a source to send a single copy of its traffic to several receivers. The IP network is then responsible for optimally replicating the traffic as close as possible to the receivers, by creating multicast distribution trees. Multicast routers are responsible for replicating data packets at the nodes of the distribution trees, i.e. at points where the path to the recipients diverges.

IP multicast is increasingly deployed, both on the Internet and in private networks, to provide multimedia content broadcasting services requiring data to be broadcast simultaneously to a set of subscribers. For example, IP multicast is particularly well suited to the transmission of radio and television programs, event broadcasting (concerts, conferences), distance learning (tele-education or tele-training), and the distribution of information to diverse communities (press releases, stock market prices, etc.).

IP multicast therefore saves precious bandwidth and network capacity resources in an IP network. It also lightens the load on broadcast applications, which no longer have to send as many copies of the program to be broadcast as they have recipients. For network operators, IP multicast offers the prospect of optimized use of network transmission and switching resources. For service providers, it will enable them to reach a wide audience, sensitive to quality of service.

Many different protocols need to be activated in an IP network to implement a multicast service. They cover...