Script: IP Multicast audio primer

IP Multicast is a bandwidth-efficient method for broadcasting data, in particular streaming audio and video, across a network. In this primer we'll discuss how IP Multicasts works, its pros and cons, and give a few examples of how the technology can be applied.

There are basically two methods of transmitting streaming data across the Internet: unicast and multicast. Unicast is the most prevalent but also uses the most bandwidth. In the unicast model, a server must deliver a stream of data for each client that wants to participate in the broadcast. For instance, a video that requires 100 kilobytes of bandwidth for a single user would require 100 times that amount for 100 users, or a total of 10 megabytes of bandwidth. This can lead to network congestion and increased server load that will decrease the overall quality of the broadcast.

But with IP Multicast, a single stream is sent from the server across the network that serves all users who want to receive the broadcast. In the prior example, this would mean the server could send out a single 100-kilobyte stream that can be picked up by each of the 100 clients, reducing the load on the network and servers significantly. The work is done by IP Multicast enabled routers, which send one stream to each router connected to it. This is done all the way through the network until the signal reaches the clients that want to receive the broadcast.

Unfortunately, there is no one protocol or standard that defines how IP Multicast should work. Since its introduction in 1988, there have been a number of proposals and protocols developed to handle multicast traffic on the network. One of the most popular specifications is Protocol Independent Multicast or PIM. PIM was developed by Cisco and is now on the standards track with the IETF.

PIM itself comes in two basic flavors: dense mode and sparse mode. Dense mode floods the network tree with a broadcast so that every branch receives the signal. Branches that do not have clients requesting the signal are then pruned from the broadcast on the fly. Sparse mode works in the opposite fashion. Only those branches with clients requesting the broadcast actually receive the stream. The request from the client goes up the network tree until a multicast connection is made between client and server. Multicast broadcasts are announced typically announced like anything else, through e-mail or on a Web site. There are some proprietary protocols such as Session Announcement Protocol (SAP) that allow server to advertise what programs they have available.

Dense mode is good for networks with plenty of bandwidth and users who are closely distributed on the network. Sparse mode is better suited for networks with users that are more dispersed.

Possible applications include live streaming media, for example, company addresses by the CEO. But IP multicasting can also be used to deliver time-sensitive data to large numbers of users and sites. For example, brokerages could use IP multicast to ensure that all stock traders get information at the same time. With a traditional unicast system, traders at the beginning of the queue would get notified first, giving them a potentially unfair advantage.

As with most things in life, IP Multicast is not perfect. For it to work, every router in a network must be enabled for it. This could pose some problems on a corporate network in need of router upgrades. But it means potentially major headaches for broadcasts across the public Internet. Not every ISP - or extranet partner - will have multicast-enabled networks. And even if they do, they may support different versions of it.

Also, multicast comes with reliability issues. By its nature, multicast is a fire-and-forget technology, as it does not receive performance information back from the client. This can make it difficult to troubleshoot problems. Some servers that support multicasting do have the capability to receive a back-channel connection from the player, which helps with reliability. However, this also diminishes scalability as the number of back-channel connections increase.

Proponents of the technology are making strides to make multicast more widely usable. There are a number of protocols under consideration that ease the implementation of multicast across networks, such as Border Gateway Multicast Protocol (BGMP) and Multicast Address Set-Claim (MASC). Also, companies such as Digital Fountain are coming up with creative ways to add reliability to multicast without losing the benefit of scalability.

For now though, IP Multicast is better suited for internal corporate broadcasts.

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