Internet Core Protocols: The Definitive Guide

Name

Message Type

Synopsis

Specifies the specific type of message being sent (such as “query” or “report”).

Size

Four bits.

Default Value

N/A

Defined In

RFCs 1112 and 2236.

Notes

Each IGMP message is identified by a four-bit numeric code that maps against a specific type of service. There are a variety of message types provided in IGMPv1 and IGMPv2. Table 4.4 lists the different message types, their codes, and descriptions for their use.

Table 4.4. IGMP Message Types and Their Meanings

Code	Message Type	Description
1	Membership Query	Used by IGMPv1 and IGMPv2 multicast routers to locate the multicast groups in use by hosts on the local network.
2	v1 Membership Report	Identifies this message as an IGMPv1 Membership Report.
6	v2 Membership Report	Identifies this message as an IGMPv2 Membership Report.
7	v2 Leave Report	Used by IGMPv2 hosts to announce that they are leaving a multicast group.

Although all of the IGMP messages that are described in RFC 1112 and RFC 2236 use 1 in the version field, each of them use different codes for the message types that are generated by the Membership Reports. For example, Figure 4.8 shows an IGMP Membership Query (identifiable by the Message Type being set to 1).

Unrecognized message types should be silently ignored.

Capture Sample

In the capture shown in Figure 4.8, the Message Type field is set to 1, indicating that this message is an IGMPv2 Membership Query.

Figure 4.8. The ...