Internet Core Protocols: The Definitive Guide

IGM Message Headers

IGMP messages consist of a predefined set of fields that indicate the type of message being passed and the multicast group that the message applies to. Each “message type” is used in specific scenarios and is indicated by a field in the header. Depending on the version of IGMP in use and the type of message being sent, an IGMP message will have four or five fields, with different fields being used for the different versions of IGMP.

Table 4.2 lists all of the fields in IGMPv1 messages, along with their size (in bits) and some usage notes. For more information on these fields, refer to the individual discussions throughout this section.

Table 4.2. The Fields in an IGMPv1 Message

Field	Bits	Description
Version	4	Indicates the version of IGMP in use (always 1).
Message Type	4	Indicates the message type (such as “query” or “report”).
Unused	8	This field is unused with IGMPv1 query messages, and must be filled with zeroes.
Checksum	16	Used to store a checksum of the IGMP message.
Multicast Group	32	The 32-bit IP address of the multicast group that this message is associated with.

Table 4.3 lists all of the fields in IGMPv2 messages, along with their size (in bits) and some usage notes. For more information on these fields, refer to the individual discussions throughout this section.

Table 4.3. The Fields in an IGMPv2 Message

Field	Bits	Description
Message Type	8	Indicates the message type (such as “query” or “report”), and also uses some bits to indicate the version of the message.
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