PLANET XGS3-24042 - Manual (Page 402)

48-1
Chapter 48 IPv4 Multicast Protocol
48.1 IPv4 Multicast Protocol Overview
This chapter will give an introduction to the configuration of IPv4 Multicast Protocol. All IPs in this chapter are
IPv4.
48.1.1 Introduction to Multicast
Various transmission modes can be adopted when the destination of packet (including data, sound and video)
transmission is the minority users in the network. One way is to use Unicast mode, i.e. to set up a separate
data transmission path for each user; or, to use Broadcast mode, which is to send messages to all users in
the network, and they will receive the Broadcast messages no matter they need or not. For example, if there
are 200 users in a network who want to receive the same packet, then the traditional solution is to send this
packet for 200 times separately via Unicast to guarantee the users who need the data can get all data wanted,
or send the data in the entire domain via Broadcast. Transferring the data in the whole range of network .The
users who need these data can get directly from the network. Both modes waste a great deal of valuable
bandwidth resource, and furthermore, Broadcast mode goes against the security and secrecy.
The emergence of IP Multicast technology solved this problem in time. The Multicast source only sends out
the message once, Multicast Routing Protocol sets up tree-routing for Multicast data packet, and then the
transferred packet just starts to be duplicated and distributed in the bifurcate crossing as far as possible. Thus
the packet can be sent to every user who needs it accurately and effectively.
It should be noticed that it is not necessary for Multicast source to join in Multicast group. It sends data to
some Multicast groups, but it is not necessarily a receiver of the group itself. There can be more than one
source sending packets to a Multicast group simultaneously. There may exist routers in the network which do
not support Multicast, but a Multicast router can encapsulate the Multicast packets into Unicast IP packets
with tunnel mode to send them to the Multicast router next to it, which will take off the Unicast IP header and
continue the Multicast transmission process, thus a big alteration of network structure is avoided. The primary
advantages of Multicast are:
1. Enhance efficiency: reduce network traffic, lighten the load of server and CPU
2. Optimize performance: reduce redundant traffic
3. Distributed application: Enable Multipoint Application
48.1.2 Multicast Address
The destination address of Multicast message uses class D IP address with range from 224.0.0.0 to
239.255.255.255. D class address can not appear in the source IP address field of an IP message. In the
process of Unicast data transmission, the transmission path of a data packet is from source address routing to
destination address, and the transmission is performed with hop-by-hop principle. However, in IP Multicast
environment, the destination addresses is a group instead of a single one, they form a group address. All
message receivers will join in a group, and once they do, the data flowing to the group address will be sent to
the receivers immediately and all members in the group will receive the data packets. The members in a