Overview of Traffic Types

Before getting too far ahead, let's first discuss the three methods used to send data: unicasts, broadcasts, and multicasts.

Unicasts

Unicasts are the most common form of communication because most traffic that is generated is sent to a specific machine, such as accessing a Web page or sending an email to an SMTP server.

With unicasts, a separate packet must be sent to each destination. In a shared environment, every network device on the segment will see the packet, but only the actual destination will process it. In a switched environment, only devices on the source and destination segments will actually see the frame.

In an environment in which the server must send the same information to numerous clients, a scalability problem is created especially if the information is a live video feed. As an example, assume that a video server will be sending a 1Mbps video feed to its participating clients. If 100 clients are participating, the video server would have to generate 100 1Mbps video feeds, totaling 100Mbps of bandwidth. Because of this scalability problem, unicasting is typically used in environments where very few end stations need to see the information from the server. Unicasting is also used when two or more clients need different kinds of information from the same server.

Broadcasts

To solve the scalability problem of unicasts, you could use broadcasts to disseminate the server's information to all the participating clients.

When a broadcast packet is generated, everyone in the broadcast domain will see this packet and process it.

In a broadcast design, the server generates a single packet that every client will see. One advantage of this approach is that no matter how many clients are participating in the application, the server generates only one feed. Using the previous example of 100 clients, only 1Mbps of bandwidth is created to generate the video feed.

Unfortunately, broadcasts have some downsides. Because of the way broadcasts are implemented, they traverse every segment in a VLAN. Also, when an end station receives this broadcast, the NIC assumes that the frame is to be processed, even if the application isn't running on the end station, which affects its CPU cycles. Another problem is that if the server is in one broadcast domain and the clients are in another, an RP, by default, will not forward the broadcast traffic. Recall that an RP is a Layer 3 device with a route processor.

Multicasts

Because of the issues associated with unicasts and broadcasts for disseminating information to many end stations, the recommended approach is to use multicasts.

When a multicast frame is generated, everyone in the broadcast domain will see this packet, but only a group of machines those running that multicast application will process it. Multicasting is the transmission of a packet to a host group, which can contain from zero to many end stations. Like broadcasts, they are sent with a best effort reliability there's no guarantee that all the machines will see the multicast.

A multicast combines the advantages of broadcasts and unicasts. The server generates only a single data feed that will be received by all the end stations, thus reducing the bandwidth impact of your network. As with broadcasts, the server has no idea as to which end stations are actually participating and their addresses. To reduce the impact of processing cycles that broadcasts generate on an end station, multicasts give the NIC the capability to determine, at Layer 2, whether to send the frame to the CPU for further processing or to discard it. This creates a more user-friendly environment for the end station. Like unicasts, multicast traffic can be intelligently routed to only those segments that have participating end stations.

Here are some important characteristics of multicast traffic:

• Capability to send traffic from zero to an infinite number of end stations.

• Designed to accommodate clients dynamically joining and leaving the multicast application.

• Allows an end station to simultaneously participate with multiple multicast applications.

• Like broadcasts, multicasts provide a best effort delivery of information; there's no guarantee that all the server information will get to the end station.

• Uses a separate multicast address for each multicast application.

With TCP/IP, multicasting is implemented using UDP. UDP provides no error correction, no flow control, and no reliability unlike TCP. However, UDP has much less overhead in its packet header, making it a more efficient protocol to use when disseminating a large multicast stream of data.

To receive multicast information, a device joins a multicast group. A multicast group is a loose grouping of devices that want to receive the same information. The membership of the group is dynamic end stations can come and go as they choose. When an end station joins a particular multicast group, it processes traffic sent to the destination multicast address. When an end station leaves a multicast group, it ignores multicast information sent to it for the old group.