3Com 10014299 manual IP Multicast Packet, Application

IP Multicast Packet Forwarding 503

	The transmitting end is first registered at the RP if it needs to send data to a
	specific address, and then sends the data to the RP. Once data reaches the RP,
	multicast data packets are duplicated and sent to receivers who are interested in
	getting them along the distribution tree path. The duplication only occurs at the
	crotch of the distribution tree. This process can automatically repeat until the data
	packets finally arrive at the destination point.
IP Multicast Packet	In the multicast model, the source host sends information to any host group
Forwarding	represented by the multicast group addresses in the destination address segment
	of the IP information packet. In contrast to the unicast model, the multicast model
	cannot base forwarding decisions on the destination addresses contained in the
	information packet. Instead, it must forward the multicast information packet to
	multiple external interfaces to send it to all the receiving sites. Therefore, the
	multicast forwarding process is more complicated than the unicast forwarding
	process.
	To guarantee that all the multicast information reaches routers by the shortest
	route, the multicast model must use the unicast routing table or the independent
	multicast routing table and check the multicast information packet receiving
	interfaces. This checking mechanism is the basis for most multicast routing
	protocols to carry out the multicast forwarding reverse path forwarding (RPF)
	check. The multicast module checks the source address in the received multicast
	data packet. If the active tree is adopted, this source address is that of the host
	sending the multicast data packet. If the shared tree is adopted, this source
	address is the root address of the shared tree. Thus, the multicast module can
	determine whether the input interface of the arrived data packet is on the shortest
	path from the receiving site to the source address. When the multicast data arrives
	at the router, if the examination has passed, the information packet is forwarded
	according to the multicast forwarding items. Otherwise, the information is
	discarded.
IP Multicast	IP multicast allows the internal data of the company to be distributed to a large
Application	number of subscribers. For example, for a company with many chain stores,
	multicast can be used to send its price information to the cash register in each
	chain store. The real-time information can be sent to multicast subscribers by
	media over the Internet, such as the current remote employee management and
	education.
	The traditional data broadcast is based on the broadcast transmission form, which
	requires much Internet bandwidth. Using multicast technology, TV and wireless
	sites can not only multicast data to Internet subscribers who really need them, but
	can also reduce the cost of network maintenance to a large extent.