Chapter 7
Internet Group Management Protocol
(IGMP)
Topics
Introduction to IP Multicast and IGMP
 IP Multicasting Overview
 Host Support
 Sending IP Multicast Traffic
 Receiving IP Multicast Traffic
 Router Support
 The Multicast-Enabled IP Internetwork
 The Internet’s Multicast-Enabled Backbone
Topics
IGMP Message Structure
 IGMP Version 1 (IGMPv1)
 IGMP Version 2 (IGMPv2)
 IGMP Version 3 (IGMPv3)
IGMP in Windows Server 2008 and Windows
Vista
 TCP/IP Protocol
 Routing And Remote Access Service
Introduction to IP Multicast and IGMP
IP Multicasting Overview
■ All multicast traffic is sent to a class D address in the range 224.0.0.0 through
239.255.255.255 (224.0.0.0/4). All traffic in the range 224.0.0.0 through 224.0.0.255
(224.0.0.0/24) is for the local subnet and is not forwarded by routers. Multicast-enabled
routers forward multicast traffic in the range 224.0.1.0 through 239.255.255.255 with an
appropriate Time to Live (TTL).
■ A specific multicast address is called a group address.
■ The set of hosts that listen for multicast traffic at a specific group address is called a
multicast group or host group. Multicast group members can receive traffic to their unicast
address and the group address. Multicast groups can be permanent or transient. A permanent
group is assigned a well-known group address. An example of a permanent group is the allhosts multicast group, listening for traffic on the well-known multicast address of 224.0.0.1.
The membership of a permanent group is transient; only the group address is permanent.
■ There are no limits on a multicast group’s size.
■ A host can send multicast traffic to the group address without belonging to the
multicast group.
■ There are no limits to how many multicast groups to which a host can belong.
■ There are no limits on when members of a multicast group can join and leave a
multicast group.
■ There are no limits on the location of multicast group members.
IP multicast must be supported by the hosts and the routers of an IP internetwork
Introduction to IP Multicast and IGMP
Host Support
■ Level 0 No support for sending or receiving IP multicast traffic
■ Level 1 Support for sending IP multicast traffic
■ Level 2 Support for sending and receiving IP multicast traffic
You can also use the following registry value:
Introduction to IP Multicast and IGMP
Sending IP Multicast Traffic
Table 7-1 Recommended Values of the TTL for IP Multicast Traffic
Introduction to IP Multicast and IGMP
Sending IP Multicast Traffic
Table 7-1 Recommended Values of the TTL for IP Multicast Traffic
Receiving IP Multicast Traffic
■ Informs the Network Interface Layer technology to add the MAC-level multicast
address that corresponds to the group address to the list of interesting destination MAC
addresses.
■ If the group address is not in the range 224.0.0.1 through 224.0.0.255 (224.0.0.0/24),
the IP module sends an IGMP Host Membership Report message to inform local routers
to forward the host group traffic to the subnet of the listening host.
Introduction to IP Multicast and IGMP
Router Support
The Multicast-Enabled IP Internetwork
Figure 7-1 A multicast-enabled intranet showing multicast-enabled hosts and routers
Introduction to IP Multicast and IGMP
Router Support
The Internet’s Multicast-Enabled Backbone
The portion of the Internet that is IP-multicast-enabled is known as the multicast
backbone (MBONE). The MBONE was originally created to multicast the audio for
Internet Engineering Task Force (IETF) meetings for members who could not attend.
Today, the MBONE is used for the audio and video of IETF meetings, launches of the
National Aeronautic and Space Administration (NASA) space shuttle, and
teleconferences of all kinds. The MBONE is also the test bed for the development of IP
multicast applications, tools, and routing protocols.
The MBONE is a logical IP multicast topology overlaid on the Internet’s physical unicast
topology. Not all Internet service providers (ISPs) support the forwarding of IP multicast
traffic. To connect two portions of the Internet that support IP multicast traffic, IP
multicast traffic is tunneled or wrapped with another IP header addressed from one
router to another router. The typical tunneling is called IP-in-IP tunneling and is
described in RFC 1853. The MBONE is a series of multicast-enabled islands connected
together with IP-in-IP tunnels.
IGMP Message Structure
Figure 7-2 IGMP message structure showing the IP header and Network Interface Layer header and trailer
IGMP Message Structure
IGMP Version 1 (IGMPv1)
 Host Membership Report
 Host Membership Query
 IGMPv1 Message Structure
Figure 7-3 The structure of an IGMPv1 message
IGMP Message Structure
IGMP Version 2 (IGMPv2)




The Leave Group Message
The Group-Specific Query Message
The Multicast Querier
IGMPv2 Message Structure
IGMP Message Structure
IGMPv2 Message Structure
Figure 7-4 The structure of an IGMPv2 message
IGMP Message Structure
IGMP Version 3 (IGMPv3)
 IGMPv3 Host Membership Query
Figure 7-5 The structure of the IGMPv3 Host Membership Query message
IGMP Message Structure
IGMP Version 3 (IGMPv3)
 IGMPv3 Host Membership Report
Figure 7-6 The structure of the IGMPv3 Host Membership Report message
IGMP Message Structure
IGMP Version 3 (IGMPv3)
 IGMPv3 Host Membership Report
Figure 7-7 The structure of the IGMPv3 Host Membership Report message group record
IGMP in Windows Server 2008 and Windows Vista
 TCP/IP Protocol
 Routing And Remote Access Service
 Interfaces in IGMP Router Mode
 Interfaces in IGMP Proxy Mode
IGMP in Windows Server 2008 and Windows Vista
 Routing And Remote Access Service
 Interfaces in IGMP Proxy Mode
Figure 7-8 The use of IGMP router mode and proxy mode
Summary
IGMP provides a mechanism for hosts to register their interest
in receiving IP multicast traffic sent to a specific group
address (the Host Membership Report message), for hosts to
indicate that they are no longer interested in receiving IP
multicast traffic sent to a specific group address (the Leave
Group message), and for routers to query the membership of
all hostgroups (the General Host Membership Query) or a
single host group (the Group-Specific Host Membership
Query). TCP/IP for Windows Server 2008 and Windows Vista
supports IGMPv1, IGMPv2, and IGMPv3, as well as IP
multicast forwarding. In Windows Server 2008, the Routing
and Remote Access service uses the IGMP routing protocol
component and interfaces in IGMP router and proxy mode to
maintain the IP multicast forwarding table and provide
multicast forwarding in limited configurations.
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