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Chapter 16
Multicast Networking

C. Kenneth Miller

In a business environment where efficient information delivery is critical to staying competitive, multicast network technology offers a new method of electronic distribution. Multicast networking enables group-oriented applications such as videoconferencing, dataconferencing, electronic software distribution, and database updates.

INTRODUCTION

Multicast transmission is the sending of one message to many, but not all, receivers. Multicast network infrastructures are becoming available in all kinds of data networks, including wide area, satellite, and wireless. This new infrastructure is being used for group-oriented data networks.

Broadcast transmission is the sending of one message to all receivers and has been used extensively in LAN environments. Broadcast traffic over WANs should be avoided, however, because it can flood the WAN with unwanted traffic, or broadcast storms.

Multicast provides the mechanism for one-to-many transmission over WAN without creating broadcast storms. Multicast network infrastructures can be created at layer 2 (i.e., the link layer) or at layer 3 (i.e., the network layer). The primary layer 3 multicast transport technique is multicast IP, which many router vendors support. Layer 3 multicast transport is independent of the underlying network architecture.

Different physical and link layer architectures support multicast and broadcast services. For example, satellite data transmission is a broadcast architecture that easily supports multicast services. Other network infrastructures are multicast LANs, multicast frame relay, and multicast SMDS.

MULTICAST LANS

Every station on the LAN listens to all transmissions. Nodes on a LAN have 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.


Exhibit 1.  One-way Multicast.

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 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 LAN and preventing broadcast storms.

Multicast Frame Relay

Frame relay is a layer 2 protocol designed for use over WAN. Frame relay is a connection-oriented protocol, which means that it emulates actual physical links with PVCs and SVCs. PVCs and SVCs represent point-to-point connections with 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 1.) A multicast server in the network maps the multicast data link connection identifiers to the individual data link connection identifiers. Individual DLCIs are also present from the members of the group to the multicast transmitter.

Two-Way Multicast

Two-way multicast enables migration of old IBM SDLC multidrop configurations to a Frame relay environment. (See Exhibit 2.)


Exhibit 2.  Two-way Multicast.


Exhibit 3.  N-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 3) was designed for teleconferencing applications.

Multicast Switched Multimegabit Data Service

SMDS was developed by Bellcore for LEC to provide high-speed connectivity between LANs in a metropolitan area. SMDS is a subset of the IEEE 802.6 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 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 intranets.

Class D Addressing

Multicast IP uses Class D IP addresses, as shown in Exhibit 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.

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 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.


Exhibit 4.  IP Address Types.


Exhibit 5.  IGMP Dialog.

The IGMP dialog is shown in Exhibit 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.

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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