System information

Troubleshooting TCP/IP 7-99
IP Multicast
IP Multicast
The Internet Protocol suite was designed for communications between two computers using unicast
addresses (that is, an address specifying a single network device). To send a message to all devices
connected to the network, a single network device uses a broadcast address. These two forms of
addressing have until now been sufficient for transferring traditional data (such as files and virtual
terminal connections).
Now that application developers are trying to deliver the same data (such as the audio and video
required for conferencing) to some, but not all, devices connected to the network, another form of
addressing is required. The new form of addressing is called multicast addresses, and it involves the
transmission of a single IP datagram to multiple hosts. This section describes the following
techniques for supporting IP multicast addresses:
UDP flooding
Subnet broadcast
Internet Group Membership Protocol
Because IP networks tend to have complex topologies with alternate paths built in for redundancy,
each technique is evaluated for its ability to deliver data without burdening the network with
duplicate packets.
UDP Flooding
UDP flooding depends on the spanning tree algorithm to place interfaces in the forwarding and
blocking states. By placing certain interfaces in the blocking state, the spanning tree algorithm
prevents the propagation of duplicate packets. The router sends specific packets (typically UDP
packets) out the interfaces that are in the forwarding state. This technique saves bandwidth by
controlling packet flow in topologies that feature redundant routers and alternate paths to the same
destination. Figure 7-9 illustrates packet flow.