Chapter 5. Introduction to IP Multicast Routing
Multicasting is the process of sending data to a group of receivers. It might be argued that unicasting and broadcasting are subsets of multicasting. In the case of unicasting , there is only a single member of the group; in the case of broadcasting, all possible receivers are members of the group. This chapter demonstrates why such an argument is valid only on a conceptual level; in networking, at least, distinct differences exist between multicasting, unicasting, and broadcasting. The delivery of radio and television programming is commonly called "broadcasting," but in reality it is multicasting. A transmitter sends data on a certain frequency, and some group of receivers acquires the data by tuning in to that frequency. The frequency is, in this sense, a multicast address. All receivers within the range of the transmission are capable of receiving the signal, but only those who listen to the correct frequency actually receive it. The signal range brings up another important concept: Radio and television transmissions have scope ”they are limited by the power of the transmitter. Receivers outside the scope of the transmission cannot receive the signal. You will see in this chapter that IP multicast networks also can have scope. You have already had some exposure to IP multicasting in Volume I. RIP-2, EIGRP, and OSPF all employ multicasting for efficiency in communicating routing information. Applications can use multicasting for exactly the same reason ”to increase network efficiency and conserve network resources. Figure 5-1 depicts a set of IP hosts. One of the hosts is a source (S) of data that must be delivered to a group (G) of receivers. There is more than one receiver, but the group does not contain all possible receivers. Figure 5-1. The Source Must Deliver the Same Data to Multiple Receivers One approach is for the source to use a replicated unicast. That is, the source creates a separate packet containing identical data for each destination host in the group. Each packet is then unicast to a specific host, as shown in Figure 5-2. Figure 5-2. Unicasting the Same Data to Multiple Receivers Places a Burden on the Source If there are only a few destinations, this scheme works fine. In fact, many "multicast" applications in use today actually utilize replicated unicast. As the number of recipients grows into the hundreds or thousands, however, the burden on the host to create and send so many copies of the same data also increases . More importantly, the host's interface, directly connected medium, directly connected router, and slow WAN links all become potential bottlenecks. There are also problems if the data is delay-sensitive and cannot be contained in a single packet. If all the copies of packet number 2 must wait for all the copies of packet number 1 to be queued and sent, the queuing delay can introduce unacceptable gaps in the data stream. Another possible approach to multicasting is to broadcast the data as depicted in Figure 5-3. This removes the burden from the source and its local facilities, which now have to send only a single copy of each packet, but it can extend the burden to the other hosts in the network. Each host must accept a copy of the broadcasted packet and process the packet. It is only at the higher layers , or possibly within the application itself, that disinterested hosts recognize that the packet is to be discarded. If the number of hosts in the receiving group is small in relation to the total number of hosts in the network, this processing burden can again be unacceptable. Figure 5-3. Broadcasting Data Can Place a Burden on the Rest of the Network NOTE When there are relatively few group members in relation to the total number of hosts in a multicast domain, the domain is sparsely populated . You will encounter this concept again later in this chapter. Another difficulty with broadcasting is that IP routers do not forward packets to broadcast destinations. If the cloud in Figure 5-3 is a routed internetwork rather than a single broadcast medium, broadcast packets cannot reach the remote hosts. Directed broadcasts could be used, but that may be the worst possible solution. Not only would all hosts receive the packet, but also the source would again be burdened with having to replicate packets. Multicasting allows the source to send a single packet to a single multicast destination address, thus removing the processing burden of replicating packets. Any receiver that is listening for the multicast address can receive the packet, removing the need for disinterested hosts to process an unwanted packet. And unlike broadcast packets, multicast-aware routers can forward multicast packets. Many aspects of IP multicasting are not covered in this chapter. This book is concerned only with IP routing, so the primary focus of this chapter is on IP multicast routing. Other topics are touched upon only as they pertain to routing. For a complete treatment of IP multicast, have a look at the references cited at the end of the chapter in "Recommended Reading." |
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