like broadcasting in the television world, where one stream goes to multiple users simultaneously. Say, for instance, that you are in San Francisco, and you want to do a videoconferencing session that involves three offices located in London. In unicast mode, you need three separate IP connections to London from the conferencing point in San Francisco. With multicast, however, you need only one IP connection. A multicast router (mrouter) enfolds your IP packets in special multicast packets and forwards those packets to an mrouter in London; in London, that mrouter removes the IP packets, replicates them, and then distributes them to the three locations in London. In IP networks, the IP mul- ticast protocol involves the use of a special group of addresses known as multicast addresses (as discussed later in this chapter). Each destination address identifies a group of receivers to which all information is replicated. Anycast A new development in IP routing is the introduction of anycast, which refers to the point-to-point flow of packets between a single client and the nearest destination server identified by an anycast address. As shown in Figure 8.11, in anycast, clients send packets addressed to A. Routers forward packets to the closest server with the address A. As with multicast, anycast involves a one-to-many association in which Chapter 8 The Internet and IP Infrastructures Anycast Server Client 2 Router 2 Router 3 IP Network Router 1 Client 1 Router 4 Anycast Router 5 Server Figure 8.11 Anycast each destination address identifies a group of receivers. However, at any given time, only one of them is chosen to receive information from any given sender. The type of anycast is determined by the criteria for selecting the nearest server. In IP anycast, the network topology is the key criterion. In Application any- cast, applications or server metrics-such as number of connections or available capacity-are the key criteria. Anycast has several advantages. First, it offers a reduction in router and link resources 