TCP IP Illustrated

Membership in this group is never reported 134 an

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Unformatted text preview: we also indicated that the router's IGMP query is sent to the destination IP address of 224.0.0.1. This is called the all-hosts group address. It refers to all the multicast-capable hosts and routers on a physical network. Each host automatically joins this multicast group on all multicast-capable interfaces, when the interface is initialized. Membership in this group is never reported. 13.4 An Example Now that we've gone through some of the details of IP multicasting, let's take a look at the messages involved. We've added IP multicasting support to the host sun and will use some test programs provided with the multicasting software to see what happens. First we'll use a modified version of the netstat command that reports multicast group membership for each interface. (We showed the standard netstat -ni output for this host in Section 3.9.) In the following output we show the lines corresponding to multicast groups in a bold font: sun % netstat -nia Address Name Mtu Network le0 1500 140.252.13. 140.252.13.33 224.0.0.1 08;00:20:03:f6:42 01:00:5e:00:00:01 140.252.1.29 sl0 552 140.252.1 224.0.0.1 127.0.0.1 lo0 1536 127 224.0.0.1 lpkts 4370 lerrs Opkts 0 4924 Oerrs Coll 0 0 13587 0 15615 0 0 1351 0 1351 0 0 The -n option prints IP addresses in numeric format (instead of trying to print them as names), -i prints the interface statistics, and -a reports on all configured interfaces. The second line of output for le0 (the Ethernet) shows that this interface belongs to the multicast group 224.0.0.1 ("all hosts"), and two lines later the corresponding Ethernet address is shown: 01:00:5e:00:00:01. This is what we expect for the Ethernet address, given the mapping we described in Section 12.4. We also see that the other two interfaces that support multicasting, the SLIP link sl0 and the loopback interface lo0, also belong to the all-hosts group. We must also show the IP routing table, as the normal routing table is used for multicast datagrams. The bold entry shows that...
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