Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 261
IGMP Header

The IGMP router places queries to its subnets and the hosts that belong to a group specified in the query response report. The IGMP headers for version 1 and version 2 are shown in the slide (you will find both types on your network). The Type field can be one of four types:

0x11: Membership Query

0x16: Version 2 Membership Report

0x17: Leave Group—for IGMP version 1 compatibility (explained later in the section)

0x12: Version 1 Membership Report—for backward compatibility

IGMP version 2 has a different header than version 1. The Version and Type fields of a version 1 header are combined into one field called the Type field. To allow a multicast router to determine the difference between the two, a new Type field was created for Version 2—Membership Report Message. IGMP version 1 and version 2 routers may coexist. An IGMP version 2 router must be able to act as a version 1 IGMP router. To determine this, the Max Response Time field is set to 0 in all queries (this maps to the unused field in version 1).

The membership query packet has two types: General Query (used to learn group members of any group on an attached network) Group Specific Query (used to learn members of a specific group).

How do you tell the difference between the two? This is determined by the group address in the IGMP header. A general query uses all 0s in the Group Address field and the specific query uses the exact group address. Both of these messages are sent using the IP header address 224.0.0.1 and a mapped multicast MAC address 01-00-5E-00-00-01 (review the conversion method, explained previously).

The Leave Group message is new to IGMP version 2. It allows a router to immediately determine if there are any members of a group left on its interface that received the Leave Group message. This is explained in a moment.

IGMP Header