The Advantages Of Multicast

The Advantages of Multicast

Any form of network communication involving the transmission of information to multiple recipients can benefit from the bandwidth efficiency of multicast technology. Examples of applications involving one-to-many or many-to-many communications include: video and audio broadcasts, videoconferencing/collaboration, dissemination of stock quotes and news feeds, database replication, software downloads, and Web site caching.

To understand the efficiency of multicasting, consider a video server offering a single channel of content, as shown in Figure 1. For full-motion, full-screen viewing, a video stream requires approximately 1.5 Mbps of server-to-client bandwidth. In a unicast environment, the server must send a separate video stream to the network for each client (this consumes 1.5 x n Mbps of link bandwidth where n = number of client viewers). With a 10-Mbps Ethernet interface on the server, it takes only six or seven server-to-client streams to completely saturate the network interface. Even with a highly intelligent Gigabit Ethernet interface on a high-performance server, the practical limit would be from 250 to 300 1.5-Mbps video streams. Therefore, the server interface capacity can be a significant bottleneck, limiting the number of unicast video streams per video server. Replicated unicast transmissions consume a lot of bandwidth within the network, which is another significant limitation. If the path between server and client traverses h3 router hops and h2 switch hops, the "multi-unicast" video would consume 1.5 x n x h3 Mbps of router bandwidth, plus 1.5 x n x h2 Mbps of switch bandwidth. With 100 clients separated from the server by two router hops and two switch hops as shown in Figure 1, a single multi-unicast channel would consume 300 Mbps of router bandwidth and 300 Mbps of switch bandwidth. Even if the video stream bandwidth is scaled back to 100 Kbps (which provides acceptable quality in smaller windows on the screen), the multi-unicast would consume 20 Mbps of both router and switch bandwidth.

Figure 1: Video Transmission in Unicast and Multicast Networks

In a multicast environment, the video server needs to transmit a single video stream for each multicast group, regardless of the number of clients that will view it. The video stream is then replicated as required by the network's multicast routers and switches to allow an arbitrary number of clients to subscribe to the multicast address and receive the broadcast. In the router network, replication occurs only at branches of the distribution tree, so essentially all of the replication occurs at the last switch hop. In the multicast scenario, only 1.5 Mbps of server-to-network bandwidth is utilized leaving the remainder free for other uses or additional channels of video content. Within the network, the multicast transmission offers similar efficiency, consuming only 1/nth of the bandwidth of the multi-unicast solution (for example, 3 Mbps of router and switch bandwidth in Figure 1).

Obviously, where there are large number of recipients of a replicated transmission, multicast technology makes a tremendous difference in both server load and network load, even in a simple network with a small number of router and switch hops. Additional features of multicast are beneficial in specific applications such as financial services. Multicast transmissions are delivered nearly simultaneously to all members of the recipient group. The variability in delivery time is limited to differences in end-to-end network delay among the range of server-to-client paths. In a unicast scenario, the server sequences through transmission of multiple copies of the data, so variability in delivery time is large, especially for large transmissions or large distribution lists. Another unique feature of multicast is that the server does not know the unicast network address of any particular recipient of the transmission--all recipients share the same multicast network address and therefore can join a multicast group while maintaining anonymity.

Primer on Multicast Technology

Multicast transmission technology is available at both the data link layer (Layer 2) and the network layer (Layer 3). For example, Ethernet, Fiber Distributed Data Interface (FDDI), and SMDS all support unicast, multicast, and broadcast MAC layer addresses. Therefore, an individual computer on these networks can simultaneously listen to a unicast address, multiple multicast addresses, and the broadcast address. Token Ring also supports the concept of multicasting but uses a different technique to address receiver groups.

If the scope of a multicast application is limited to a single physical or logical LAN, multicasting over the data link layer is sufficient. However, most multipoint applications are of interest only if their reach can be extended to a distributed campus or even wide-area environment consisting of many different networking technologies, including Ethernet, FDDI, Token Ring, Frame Relay, and ATM. For these extended environments, multicast must also be implemented at Layer 3. Multicast transmission at Layer 3 involves several special mechanisms:

Addressing--There must be a Layer 3 address that is used to communicate with a group of receivers rather than a single receiver. In addition, there must be a way of mapping this address onto Layer 2 multicast addresses of the underlying physical networks. For IP networks, Class D addresses have been set aside for multicast addressing. A Class D address consists of 1110 as the higher order bits in the first octet followed by an unstructured 28 bit group address. For mapping IP multicast addresses to Ethernet addresses, the lower 23 bits of the Class D address are mapped into a block of Ethernet addresses that have been reserved for multicast. With this mapping scheme, each Ethernet multicast address corresponds to 32 IP multicast addresses. This means that a host receiving multicasts may need to filter out unwanted multicast packets being forwarded to other groups with the same MAC layer multicast address. Ethernet multicast addresses have a "01" in the first byte of the destination address to allow the network interface

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