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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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6.2.2 IGMP operation<br />

– Filter-mode-change records are sent by a system whenever an interface’s<br />

state changes for a particular multicast address. There are two group<br />

record values which denote a filter-mode-change record:<br />

CHANGE_TO_INCLUDE_MODE: This indicates that the interface has<br />

changed to the INCLUDE filter mode for the specified multicast<br />

addresses.<br />

CHANGE_TO_EXCLUDE_MODE: This indicates that the interface has<br />

changed to the EXCLUDE filter mode for the specified multicast<br />

addresses.<br />

– Source-list-change records are sent by a system whenever an interface<br />

wants to alter the list of source addresses without altering its state. There<br />

are two group record values that denote a source-list-change record:<br />

ALLOW_NEW_SOURCES: This indicates that the interface has<br />

changed such that it wants to receive messages from additional<br />

sources. If the filter is an INCLUDE filter, the specified multicast<br />

addresses will be added. If it is an EXCLUDE filter, the specified<br />

multicast addresses will be removed.<br />

BLOCK_OLD_SOURCES: This indicates that the interface has<br />

changed such that it no longer wants to receives messages from<br />

additional sources. If the filter is an INCLUDE filter, the specified<br />

multicast addresses will be removed. If it is an EXCLUDE filter, the<br />

specified multicast addresses will be added.<br />

We discuss these group record types in greater detail in “IGMPv3 specific host<br />

operations” on page 248.<br />

Both hosts <strong>and</strong> multicast routers participate in IGMP functions.<br />

Host operations<br />

To receive multicast datagrams, a host must join a host group. When a host is<br />

multihomed, it can join groups on one or more of its attached interfaces. If a host<br />

joins the same group on multiple interfaces, the multicast messages received by<br />

the host can be different. For example, 244.0.0.1 is the group for all hosts on this<br />

subnet. Messages in this group received through one subnet will always be<br />

different from those on another subnet.<br />

Multiple processes on a single host can listen for messages from the same<br />

group. When this occurs, the host joins the group once. The host internally tracks<br />

each process interested in the group.<br />

Chapter 6. <strong>IP</strong> multicast 247

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