3Com 10014298 manual Comparison Between the Unicast and Multicast Transmission

110CHAPTER 6: MULTICAST PROTOCOL

Figure 26 Comparison Between the Unicast and Multicast Transmission

Receiver

Unicast

Receiver

Receiver

Server

Receiver

Multicast

Receiver

Server Receiver

A multicast source does not necessarily belong to a multicast group. It only sends data to the multicast group and it is not necessarily a receiver. Multiple sources can send packets to a multicast group simultaneously.

A router that does not support multicast may exist on the network. A multicast router can encapsulate multicast packets in unicast IP packets by tunneling and sending them on to the neighboring multicast router. The neighboring multicast router removes the unicast IP header and continues the multicast transmission.

Multicast advantages:

■Enhanced efficiency by reducing network traffic and relieving server and CPU loads.

■Optimized performance decreases traffic redundancy.

■Distributed applications make multipoint applications possible.

Configuring an IP Multicast Overview is described in the following sections:

■Multicast Addresses

■IP Multicast Protocols

■Forwarding IP Multicast Packets

■Applying Multicast

Multicast Addresses The destination addresses of multicast packets use Class D IP addresses ranging from 224.0.0.0 to 239.255.255.255. Class D addresses cannot appear in the source IP address fields of IP packets.

During unicast data transmission, a packet is transmitted from the source address to the destination address with the “hop-by-hop” principle of the IP network. A packet has more than one destination address in a multi-cast environment, i.e., a group of addresses. All the information receivers join a group. Once a receiver joins the group, data flowing to the group is sent to the receiver immediately. All members in the group can receive the packets. Membership of a multicast group is dynamic, that is, hosts can join and leave groups at any time.