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 Forwarding In the multicast model, the source host sends information to any host group
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 Application IP multicast allows the internal data of the company to be distributed to a large
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.