Thunder-link.com: What’s the Basic Multicast Concepts?

Basic Concepts

The following section provides an example of watching the program of a TV channel to aid the understanding of relevant concepts of IP multicast.

The multicast group is an agreement between the sender and the recipient. For example, a TV channel can be regarded as a multicast group.

The TV station is the multicast source and it sends data to a certain TV channel.

The STB is a receiving host. When the user chooses to watch the program of a channel, this action can be regarded as the host joining a multicast group.

Then, the TV set displays the program of the TV channel to the user, which means that the host has received the data sent to this multicast group.

The user can turn on or turn off the STB or switch between channels any time, which means that the host can join or leave a multicast group dynamically.

Figure 1 Multicast concept diagram .

Table 1 Table of basic concepts

Multicast group

A multicast group is identified by a multicast IP address. Any host (or any other receiving device) joining a multicast group becomes a member of the group.

The group member can identify and receive the IP messages destined to the multicast IP address.

Multicast source

A signal source sending IP messages destined to a multicast address is called a multicast source.

A multicast source can send data to multiple multicast groups at the same time.
Multiple multicast sources can send data to a multicast group at the same time.

Multicast group member

The members of a multicast group are dynamic. Hosts in a network can join or leave a multicast group any time. Multicast group members may be widely

dispersed across the network.

A multicast source is usually not a data recipient at the same time and is not the member of a multicast group.

Multicast duplication

Multicast duplication is a capability with which the network device duplicates a multicast message from an ingress port into multiple copies and sends

them to multiple egress ports. To ensure effective transmission of multitudes of data, this function can be implemented only by hardware.

Multicast address

To enable the communication between a multicast source and its members, a network-layer multicast address must be available, which is the

multicast IP address. In addition, a technology must also be available for mapping the multicast

IP address to a link-layer multicast MAC address. The following part of this section will describe the two types of multicast address.

Multicast IP address

As specified by Internet Assigned Numbers Authority (IANA), multicast messages use class-D IP addresses (224.0.0.0-239.255.255.255)

as their destination addresses, and the class-D IP addresses must not appear in the source IP address

field of the IP messages. For details of the MAC address range and description, see Table 2.

Multicast addresses are not allocated to the receiving device or the multicast source device for identifying their network location. In the case of the

multicast source device, the allocated multicast address is used for generating and carrying multicast data; in the case of the receiving device, the

multicast address is used for distinguishing multicast data.

In an actual multicast application, the multicast address does not need to be manually input. For example, in the live TV service, which is a common

application, a menu interface is provided. When the user orders a program using a remote controller, the application software will automatically obtain

the multicast IP address corresponding to the program.

Ethernet multicast MAC address

When IP messages are unicast over an Ethernet, the destination MAC addresses used are the MAC addresses of recipients. However, in the transmission

of multicast messages, the transmission destination is no longer a specific recipient. Instead, it is a group with uncertain members. In this case, the

multicast MAC address is used.

Specified by IANA, the most significant 25 bits of a multicast MAC address are 0x01005e, and the least significant 23 bits of the MAC address are the

least significant 23 bits of the multicast IP address. The following figure shows the mapping.

Figure 2 Mapping between multicast MAC address and multicast IP address

The first four bits of the multicast IP address are 1110, which stands for the multicast ID, and in the last 28 bits, only 23 bits are mapped to the MAC

address.Therefore, five bits of information in the IP address is lost. The direct result is that 32 multicast IP addresses are mapped to the same MAC

address.

Multicast router

Multicast routers support multicast.

A multicast router supports the following functions:

Manages group members in the network segment close to user hosts.
Supports multicast routing for forwarding multicast packets.
Functions as a group member.

Table2 Multicast addresses and meanings

Basic Managed Objects

Table3 Basic managed objects

Multicast VLAN

A multicast VLAN (also called an MVLAN) refers to the VLAN tag carried by multicast data. MVLANs are usually divided based on ISP. By the

implementation of the forwarding plane, control plane, and management plane based on VLAN instance, multicast services are provisioned to

the users of the same device, allowing the users not to be interfered by each other. Except the super VLAN, the VLAN of any attribute or any

type configured on the device can serve as an MVLAN. For details on the MVLAN, see "Multi-instance Multicast".

Multicast program

A multicast program can be regarded as a multicast group. Its basic attribute is the multicast IP address. The device can manage a multicast

program at a finer grain, such as by rights control and CAC.

According to whether the attributes (such as the multicast IP address) of each program are configured before the service is provisioned, multicast

programs can be classified into two types: pre-configured programs and dynamic programs. Determine whether to support the programs of the two types based on the configured MVLAN

program mapping mode. Table 4 shows the supporting status for pre-configured programs and dynamic programs. For details about dynamic programs, see Dynamic Programs.

Multicast uplink port

A multicast uplink port is one through which a multicast source connects to the device, and is also a port through which an upper-layer multicast router connects to the device.

According to their dependency on the link-layer loop protocol, multicast uplink ports can be classified into two types: manually configured (static) uplink ports and dynamic uplink ports.

For details on dynamic uplink ports, see Ring Network of Uplink Ports.

Multicast user

A multicast user is a multicast data recipient. A service stream must be configured for the multicast user for carrying multicast control messages

in the upstream direction (the device can distinguish the user by traffic classification). Therefore, a multicast user corresponds to a unique terminal

or service subscriber. In addition, an MVLAN must be specified for the multicast user to indicate to which ISP the service subscriber belongs.

Table4 Program type supported