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MULTICAST LANS

Every station on the LAN listens to all transmissions. Nodes on a LAN have media access control (MAC) addresses, which are sometimes called physical addresses because they designate a physical node on the network. MAC addresses are global, which means that each one is unique.

MAC addresses come in three varieties: individual (unicast), multicast, and broadcast. Unicast addresses identify end points in point-to-point communications. The MAC broadcast address is the all 1s address. MAC multicast addresses are primarily used for mapping to upper-layer multicast addresses.

MAC broadcast frames are usually filtered by bridges and routers, confining them to the local LAN and preventing broadcast storms.

MULTICAST FRAME RELAY

Frame relay is a layer 2 protocol designed for use over WANs. Frame relay is a connection-oriented protocol, which means that it emulates actual physical links with permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). PVCs and SVCs represent point-to-point connections with data link connection identifiers (DLCIs) and do not usually have the facility for one-to-many connections. However, the Frame Relay Forum recently released specifications for one-way, two-way, and N-way multicast services over frame relay.

One-Way Multicast

One-way multicast is suitable for electronic distribution of information from an information service provider. (See Exhibit 2-1-1.) A multicast server in the network maps the multicast DLCI to the individual DLCIs. Individual DLCIs are also present from the members of the group to the multicast transmitter.


Exhibit 2-1-1.  One-Way Multicast

Two-Way Multicast

Two-way multicast enables migration of old IBM synchronous data link control (SDLC) multidrop configurations to a frame relay environment. (See Exhibit 2-1-2.)


Exhibit 2-1-2.  Two-Way Multicast

N-Way Multicast

N-way multicast allows any node to be either a transmitter to the group or a receiver. N-way multicast (see Exhibit 2-1-3) was designed for teleconferencing applications.


Exhibit 2-1-3.  N-Way Multicast

MULTICAST SWITCHED MULTIMEGABIT DATA SERVICE

Switched multimegabit data service (SMDS) was developed by Bellcore for local exchange carriers to provide high-speed connectivity between LANs in a metropolitan area. SMDS is a subset of the IEEE 802.6 metropolitan area network (MAN) standard and is offered as a service by several carriers.

SMDS uses a connectionless network architecture in which all nodes can communicate to all other nodes on the network, similar to LANs. All packets sent on the network are available for reception by all nodes, which means that SMDS can support multicast, unicast, and broadcast services.

COMPARING BROADCAST AND MULTICAST IP

IP supports broadcast as well as multicast services. The IP address 255.255.255.255 defines the global broadcast address and is mapped into the MAC global broadcast address on the LAN to which it is attached. The IP global broadcast address is not usually forwarded by routers out of a local network.

A second category of IP broadcast address is the network broadcast address. In this case, the network portion of the Class A, B, or C IP address is sent to the desired network destination; the host portion is the all 1s broadcast address. Network broadcast addresses are forwarded by routers to the designated network, where they are mapped into that network's MAC broadcast address. For example, the Class C IP address 206.42.32.255 is the network broadcast address for network 206.42.32.

Network broadcast addresses can provide so-called directed broadcast services to a particular network or subnetwork over an internetwork.

The IP Multicast Protocol

Multicast IP is a new technology that provides network layer routing of IP Class D group address packets in transmission control protocol and Internet protocol (TCP/IP) networks. Multicast IP operates over any network architecture, and the multicast groups can be set up and torn down within seconds. Multicast IP is destined to be the dominant means for providing multicast services in data networks of all kinds including mixed network environments or internets.

Class D Addressing

Multicast IP uses Class D IP addresses, as shown in Exhibit 2-1-4. IP Class A, B, and C addresses are used for point-to-point (unicast) communications and consist of network and host components. Class D addresses, by contrast, have only one component that identifies the multicast group.


Exhibit 2-1-4.  IP Address Types

Class D addresses occupy the range from 224.0.0.0 to 239.255.255.255 and can be assigned semipermanently or temporarily for the length of time a group is in place. Groups may be set up and torn down in seconds.

Hosts that belong to a group need to support RFC 1112, which is the standard for host extensions for IP multicasting. RFC 1112 specifies the Internet group management protocol (IGMP) that is used by members of a multicast group to inform the nearest router supporting multicast routing of their presence in a particular group. The router is responsible for updating routing tables so that multicast packets with the Class D address associated with that group are forwarded to the subnetwork that includes members of the group.

The IGMP dialog is shown in Exhibit 2-1-5. Two messages are provided in the dialog: the IGMP query and the IGMP response. Queries are sent to the all hosts' multicast group address— 224.0.0.1—and carry an IP time-to-live of 1. Hosts respond to queries with host membership reports for each host group to which they belong.


Exhibit 2-1-5.  IGMP Dialog

There are two techniques for reducing the number of concurrent reports and the total number of reports transmitted. A host can start a report delay timer or it can send a report that other members of the same group on the network can overhear.


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