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Unformatted text preview: deed, a multicast router
doesn't even care which host belongs to a group. It only needs to know that at least one host belongs to a
group on a given interface.
On a single physical network without any multicast routers, the only IGMP traffic is the reports issued by the
hosts that support IP multicasting, when the host joins a new group.
In Figure 13.3 we noted that the TTL field of the reports and queries is set to 1. This refers to the normal TTL
field in the IP header. A multicast datagram with an initial TTL of 0 is restricted to the same host. By default,
multicast datagrams are sent with a TTL of 1. This restricts the datagram to the same subnet. Higher TTLs
can be forwarded by multicast routers.
Recall from Section 6.2 that an ICMP error is never generated in response to a datagram destined to a
multicast address. Multicast routers do not generate ICMP "time exceeded" errors when the TTL reaches 0.
Normally user processes aren't concerned with the outgoing TTL. One exception, however, is the Traceroute program (Chapter
8), which is based on setting the TTL field. Since multicasting applications must be able to set the outgoing TTL field, this
implies that the programming interface must provide this capability to user processes. By increasing the TTL an application can perform an expanding ring search for a particular server. The first
multicast datagram is sent with a TTL of 1. If no response is received, a TTL of 2 is tried, then 3, and so on. file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/igmp_int.htm (3 of 7) [12/09/2001 14.47.02] Chapter 13. IGMP: Internet Group Management Protocol In this way the application locates the closest server, in terms of hops.
The special range of addresses 18.104.22.168 through 22.214.171.124 is intended for applications that never need to
multicast further than one hop. A multicast router should never forward a datagram with one of these
addresses as the destination, regardless of the TTL.
In Figure 13.3...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12