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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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The fields in the IGMP message contain the following information:<br />

► Type: This field specifies the type of IGMP packet:<br />

– 0x’11’: Specifies a membership query packet. This is sent by a multicast<br />

router. There are two subtypes of membership query messages:<br />

242 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

General Query: This is used to learn which groups have members on<br />

an attached network.<br />

Group-Specific Query: This is used to learn if a particular group has<br />

any members on an attached network.<br />

– 0x’12’: Specifies an IGMPv1 membership report packet. This is sent by a<br />

multicast host to signal participation in a specific multicast host group.<br />

– 0x’16’: Specifies an IGMPv2 membership report packet.<br />

– 0x’17’: Specifies a leave group packet. This is sent by a multicast host.<br />

► Max resp time: This field is used in membership query messages. It specifies<br />

the maximum allowed time a host can wait before sending a corresponding<br />

report. Varying this setting allows routers to tune the leave latency. This<br />

references the time between the last host leaving a group <strong>and</strong> the time the<br />

routing protocol is notified that there are no more members.<br />

► Checksum: This field contains a 16-bit checksum.<br />

► Class D Address: This field contains a valid multicast group address. It is<br />

used in a report packet.<br />

IGMPv3 messages<br />

The IGMPv2 message format has been extended in IGMP version 3, defined in<br />

RFC 3376 (which obsoletes RFC 2236). Version 3 allows receivers to subscribe<br />

to or exclude a specific set of sources within a multicast group. To accommodate<br />

this, The 0x’11’ type membership query packet has been altered, <strong>and</strong> a new<br />

IGMP packet type of 0x’22’ has been added. However, all IGMPv3<br />

implementations must still support packet types 0x’12’, 0x’16’, <strong>and</strong> 0x’17’.

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