The easiest way to begin to understand multicasting is to compare it against two other address types and their communication models.

Unicast Model

A unicast address is designed to transmit a packet from a source to a single destination. Unicast transmissions are for one-to-one communication. If multiple users need to receive the same communication, the source operating in unicast mode generates and sends each copy separately.

Broadcast Model

A broadcast address is used to send a datagram from a source to multiple destinations - an entire subnetwork, for example. Broadcast transmissions produce one-to-many communication, but some of the receivers may not want or need to receive the communication.

Multicast Model

A multicast address is used for one-to-many and many-to-many communication in an environment where users and network devices either explicitly or implicitly communicate their desire to receive the communication.

In contrast to unicast, a source that uses IP multicast generates and sends only one copy of the information that is desired by multiple receivers. At point where the delivery path that reaches group members diverges, network devices replicate and forward the packets. This approach makes efficient use of both source processing power and network bandwidth.

When using the Internet Protocol (IP) as the basis for multicast communication, the requests for and delivery of the communication is fundamentally controlled by referencing certain IP addresses or their MAC-based equivalents. These addresses are called group addresses or groups and hosts that reference these addresses are called group members.

IP multicast group members can be scattered across multiple subnetworks; thus, successful transmission from a source to group members can occur within a campus LAN, a MAN, or over a WAN.

As an extension to the standard IP network-level protocol, IP multicast was first defined in 1985 in RFC 966. Certain other protocols are used to support IP multicast processes. These are explained later in this chapter.

Benefits of IP Multicast

New applications that are designed to increase productivity within and across organizations are driving the need for network infrastructures to support IP multicast. When the application content is time-sensitive or requires significant bandwidth (for example, a video stream), the IP multicast process provides an efficient delivery mechanism.

The business benefits of using IP multicast are that it:

• Enables the simultaneous delivery of information to many receivers in the most efficient, logical way.

• Vastly reduces the load on the source (for example, a server) because it does not have to produce multiple copies of the same data.

• Makes efficient use of network bandwidth and scales as the number of participants or collaborators expands.

• Works in concert with other protocols and services, such as Quality of Service (QoS) and Resource Reservation (RSVP) requests to support real-time multimedia.