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Introduction to IP Multicast


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Introduction to IP Multicast



A Brief History of IP Multicast



The Pros of IP Multicast



The Cons of IP Multicast



Multicast Applications



MBone--The Internet's Multicast Backbone




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Developing IP Multicast Networks, Volume I

From: Developing IP Multicast Networks, Volume I
Author: Beau Williamson
Publisher: Cisco Press (53)
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1. Introduction to IP Multicast

At one end of the IP communication spectrum is IP unicast communication, where a source IP host sends packets to a specific destination IP host. In this case, the destination address in the IP packet is the address of a single, unique host in the IP network. These IP packets are forwarded across the network from the source host to the destination host by routers. The routers at each point along the path between the source and destination use their unicast Routing Information Base (RIB) to make unicast forwarding decisions based on the IP destination address in the packet.

At the other end of the IP communication spectrum is an IP broadcast, where a source host sends packets to all IP hosts on a network segment. The destination address of an IP broadcast packet has the host portion of the destination IP address set to all ones and the network portion set to the address of the subnet (see Figure1-1). (In some cases the host portion is set to all zeros, but this form of IP broadcast address is generally no longer used.)

Figure 1-1. IP Broadcast Addresses

IP hosts (including routers) understand that packets, which contain an IP broadcast address as the destination address, are addressed to all IP hosts on the subnet. Unless specifically configured otherwise, routers do not forward IP broadcast packets and, therefore, IP broadcast communication is normally limited to the local subnet. Figure1-2 clearly illustrates this point.

Figure 1-2. IP Broadcast Being Blocked by a Router

In this example, Host A sends out a broadcast to the local subnet Because Hosts B and C are on the same subnet as Host A, they receive the broadcast. Host D, however, is on a different subnet ( and does not receive the broadcast because the router does not forward broadcasts. If routers forwarded these broadcasts, route loops are likely to cause a catastrophic broadcast storm.

If your goal is to permit a host to send IP packets to other hosts not on the local subnet, then IP broadcasting is not sufficient to accomplish this goal.

IP multicasting falls between IP unicast and IP broadcast communication and enables a host to send IP packets to a group of hosts anywhere within the IP network. To do so, the destination address in an IP multicast packet is a special form of IP address called an IP multicast group address. (The format of IP multicast group addresses and exactly how hosts become members of a multicast group are explained in Chapter 2, “Multicast Basics.”) IP multicast routers must forward incoming IP multicast packets out all interfaces that lead to members of the IP multicast group. The IP multicast group address is specified in the IP destination address field of the packet.

Exactly how the routers learn which interface to forward the packet to is part of the magic of IP multicast routing. The explanation of how this magic works is one of the goals of this book. By the time you finish reading this book, you should have a good understanding not only of how IP multicasting works in general but also of how to design efficient IP multicast networks using Cisco routers.

This chapter offers a brief history of IP multicasting, a discussion on the pros and cons of multicast, a description of various multicast applications, and an introduction to the multicast backbone.

A Brief History of IP Multicast

At Stanford University in the early 1980s, a doctoral graduate student, Steve Deering, was working on a distributed operating system project for his advisor, David Cheriton. This distributed operating system was called Vsystem and was composed of several computers tied together into a loosely coupled multiprocessing system via a single Ethernet segment. The computers on this Ethernet segment worked together and communicated at the operating system level via special messages sent on the common Ethernet segment. One of the operating system primitives permitted one computer to send a message to a group of the other computers on the local Ethernet segment using a MAC layer multicast.

As the project progressed, the need arose to add more computers to the multiprocessing system. Unfortunately, the only available computers were on the other side of the campus with production routers between the two networks. Consequently, the graduate students had to extend the operating system's inter-processor communications to work at Layer 3 of the OSI reference model so that the computers on the other side of the campus could function as part of the loosely coupled multiprocessor system. In addition, the MAC layer multicast messaging would also have to be extended to work at Layer 3. The task of finding a way to extend the MAC layer multicast capability across the Layer 3 routed network primarily fell to Steve Deering.

After studying the Open Shortest Path First (OSPF) Protocol and the Routing Information Protocol (RIP) IP routing protocols, Steve concluded that the link-state mechanisms of OSPF could certainly be extended to support multicasting. He also concluded that the basic mechanisms of RIP could be used as the basis for a new distance vector-based multicast routing protocol. This idea led to more research into the area of IP multicasting and ultimately resulted in Steve Deering's doctoral thesis, “Multicast Routing in a Datagram Network,” published in December 1991.

Dr. Deering's thesis also described a Host Membership Protocol, which became the basis for today's Internet Group Membership Protocol (IGMP) that IP multicast hosts use to signal to the router on the network that they desire to join a multicast group. In addition, Dr. Deering's thesis described a distance vector-based IP multicast routing protocol that was the basis for the Distance Vector Multicast Routing Protocol (DVMRP), also developed by Dr. Deering a few years later. These two protocols provided the first successful extensions to the IP packet network model to allow multicasting to be extended to Layer 3 of the OSI model. Since that time, advances in IP multicasting technology have continued and additional protocols such as Protocol Independent Multicasting (PIM) and multiprotocol extensions to the Border Gateway Protocol (BGP) have been developed. These protocols permit IP multicasting to scale beyond the initial limited implementations to large, enterprise-wide multicast networks and eventually on to a native, completely multicast-enabled Internet.


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