System information

Troubleshooting TCP/IP 7-101
Internet Group Membership Protocol
Whenever there is a duplicate path in the network, a duplicate packet is delivered. Because many
multicast applications are data intense, packet duplication is a significant disadvantage of subnet
broadcast.
Internet Group Membership Protocol
Internet Group Membership Protocol (IGMP), defined in RFC 1112, relies on Class D IP addresses
for the creation of multicast groups. By using a specific Class D address, an individual host
dynamically registers itself in a multicast group. Hosts identify their group memberships by sending
IGMP messages. Traffic is then sent to all members of that multicast group.
Routers listen to IGMP messages and periodically send out queries to discover which groups are
active on which LANs. To build multicast routes for each group, routers communicate with each
other using one or more of the following routing protocols:
• Distance Vector Multicast Routing Protocol
• Multicast Open Shortest Path First
• Protocol Independent Multicast
These routing protocols are discussed in the following sections.
Distance Vector Multicast Routing Protocol
Distance Vector Multicast Routing Protocol (DVMRP), defined in RFC 1075, uses a technique
called reverse path flooding. With reverse path flooding, on receipt of a packet, the router floods the
packet out all paths except the path that leads back to the source of the packet, which ensures that a
data stream reaches all LANs. If the router is attached to a LAN that does not want to receive a
particular multicast group, the router sends a “prune” message back to the source to stop the data
stream. When running DVMRP, routers periodically reflood the network to reach new hosts, using
an algorithm that takes into account the frequency of flooding and the time required for a new
multicast group member to receive the data stream.
To determine which interface leads back to the source of a data stream, DVMRP implements its own
unicast routing protocol. The DVMRP unicast routing protocol is similar to RIP and is based on hop
counts only. The path that multicast traffic follows may not be the same as the path that unicast traffic
follows.
The need to reflood prevents DVMRP (especially early versions that do not implement pruning)
from scaling well. Despite its limitations, DVMRP is widely deployed in the IP research community.
It has been used to build the multicast backbone (MBONE) across the Internet.
The MBONE is used, for example, to transmit conference proceedings and deliver desktop video
conferencing. Networks that wish to participate in the MBONE dedicate special hosts to the
MBONE. The hosts establish tunnels to each other over the IP Internet and run DVMRP over the
tunnels. The MBONE is a very high consumer of bandwidth both because of the nature of the traffic
(audio and video) and because it is implemented with host-based tunnels. Host-based tunnels tend
to result in packet duplication, which the backbone networks transmit unnecessarily.
In addition, the MBONE relies on extremely knowledgeable administrators for support. Despite
their efforts, the MBONE has caused significant disruption to the Internet when popular events or
multiple events are active.