This preview shows page 1. Sign up to view the full content.
Unformatted text preview: ts, a multicast router keeps a table of which of its interfaces have one or more
hosts in a multicast group. When the router receives a multicast datagram to forward, it forwards the
datagram (using the corresponding multicast link-layer address) only out the interfaces that still have hosts
with processes belonging to that group.
Figure 13.3 shows these two IGMP messages, reports sent by hosts, and queries sent by routers. The router is
asking each host to identify each group on that interface. file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/igmp_int.htm (2 of 7) [12/09/2001 14.47.02] Chapter 13. IGMP: Internet Group Management Protocol Figure 13.3 IGMP reports and queries.
We talk about the TTL field later in this section.
There are many implementation details in this protocol that improve its efficiency. First, when a host sends
an initial IGMP report (when the first process joins a group), there's no guarantee that the report is delivered
(since IP is used as the delivery service). Another report is sent at a later time. This later time is chosen by the
host to be a random value between 0 and 10 seconds.
Next, when a host receives a query from a router it doesn't respond immediately, but schedules the responses
for later times. (We use the plural "responses" because the host must send one report for each group that
contains one or more members.) Since multiple hosts can be sending a report for the same group, each
schedules its response using random delays. Also realize that all the hosts on a physical network receive all
the reports from other hosts in the same group, because the destination address of the report in Figure 13.3 is
the group's address. This means that, if a host is scheduled to send a report, but receives a copy of the same
report from another host, the response can be canceled. This is because a multicast router doesn't care how
many hosts belong to the group-only whether at least one host belongs to the group. In...
View Full Document
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