IP networks have supported multicast transmission for nearly 20 years, but the technology has only recently entered the realm of widespread use. As the name implies, multicasting is a technique for transmitting data from a single source to a predefined collection of recipients. This concept poses some special challenges that aren't seen in more conventional transmission techniques, such as broadcasting, in which the message is sent to all computers on a network segment, and unicasting, in which a message passes from a single source to a single recipient.

Efficient use of multicasting can significantly reduce traffic load, especially on networks that support streaming-style multimedia transmissions. Multicasting applications and technologies have received increased attention with the rise of audiovisual technologies; however, multicasting remains a mystery to many software developers, system admins, and end users who might benefit from more extensive use of this promising technique. In this article, we offer a glimpse at the practical side of multicasting, including a sample configuration that uses the free XORP routing protocol suite.

What Is IP Multicasting?

Figure 1 shows the idea behind multicast transmission. Source A generates a data stream with throughput of 1Mbps, and the data stream is received by three recipients. Figure 1a shows a unicast transmission between the source and the receivers. The transmission results in three independent, but identical, data streams, which means that a bandwidth of 3Mbps is consumed on the link between the source and distribution networks. In contrast, a multicast scenario (see Figure 1b) requires only one data stream from the source, so the load on the link is constant and independent of the number of receivers.

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