The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)

IP Routing In A Subnet Or Classless Addressing (CIDR) Environment 1 2 Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)

Multicast Group Management

Group management encompasses all of the activities required to set up groups of devices. They must be able to dynamically join groups and leave groups, and information about groups must be propagated around the IP internetwork. To support these activities, additional techniques are required. The Internet Group Management Protocol (IGMP) is the chief tool used for this purpose. It defines a message format to allow information about groups and group membership to be sent between devices and routers on the internet.

This is probably the most complicated: handling and routing datagrams in a multicast environment. There are several issues here:

• Since we are sending from one device to many devices, we need to actually create multiple copies of the datagram for delivery, in contrast to the single datagram used in the unicast case. Routers must be able to tell when they need to create these copies.
  
  
• Routers must use special algorithms to determine how to forward multicast datagrams. Since each one can lead to many copies being sent various places, efficiency is important to avoid creating unnecessary volumes of traffic.
  
  
• Routers must be able to handle datagrams sent to a multicast group even if the source is not a group member.

Routing in a multicast environment requires significantly more intelligence on the part of router hardware. Several special protocols, such as the Distance Vector Multicast Routing Protocol (DVMRP), and the multicast version of OSPF, are used to enable routers to forward multicast traffic effectively. These algorithms must balance the need to ensure that every device in a group receives a copy of all datagrams intended for that group, with the need to prevent unnecessary traffic from moving across the internetwork.

This overview has only scratched the surface of IP multicasting. The complexity involved in handling groups and forwarding messages to multicast groups is one reason why support for the feature has been quite uneven and as a consequence, it is not used widely. Another issue is the demanding nature of multicasting; it uses a great deal of network bandwidth for copies of messages, and also requires more work of already-busy routers.

IP Routing In A Subnet Or Classless Addressing (CIDR) Environment 1 2 Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)