An Introduction To Multicasting Strategies

Brad Huntting, University of Colorado
David Mertz, Gnosis Software
March 2003

How is the Internet different from other networks? Advocates tout the Internet as the network to end all networks; they claim that it is has, or can be readily adapted to have, all the capabilities needed for any conceivable application. Such claims may be true in principle--but as currently deployed a major category of networking is little utilized: multicasting.

What Is Multicasting?

Nearly all the traffic on the Internet today is unicast: it is sent by a single host and is intended to be received by a single host. Unicasting starkly contrasts with networks such as AM/FM radio in which data is broadcast through the local air waves to everyone in the immediate vicinity whether they requested it or not.

The Internet does not support broadcast. There was early discussion of a global Internet broadcast address; but network engineers quickly decided that this was a bad idea.

Somewhere between unicast and broadcast is multicast. For this article we are interested in "IP-multicast"--that is, multicasting that happens at the Internet Protocol network layer. Ethernet, for example, supports a lower-layer form of multicast. But ethernet is not suitable for global networking, where general-purpose IP-multicast is. Like broadcast, a single multicast data transmission can be received by many hosts. But unlike broadcast, the data is not sent to all corners of the Internet; rather it only goes to those networks which have specifically requested it.

We believe that that advancement and refinement of multicast technologies is essential to achieving the convergence of telecommunication and computing technologies that Intel envisions. Multicast technologies, implemented on Intel servers, PCs, handhelds, and routers will soon bring audio-video content formerly only workable on a broadcast model, to the Internet.

The Problem Multicasting Solves

To understand why the Internet needs multicasting imagine trying to use the Internet to distribute all the world's cable TV and radio. Using the unicast model, a TV station would have to send identical copies of its transmission to every single viewer on the net. With unicast, a company or organization inherently limits its own popularity.

One might naively think that the extra server and bandwidth requirements of extra users could be addressed just by buying more servers and more bandwidth. But that hunch fails to understand the magnitudes involved. Suppose--optimistically-- that a video signal can be compressed into a dual-ISDN 128 Kbps stream. An OC3 line is quite comfortably more than this; at 155 Mbps, you can squeeze over a thousand such unicast streams on such a channel. Now imagine you have a million users--a realistic number for for a popular TV broadcast. You are not just out of luck, you are three orders-of-magnitude out of luck!

With unicast, the cost of transmission is O(n) for n receivers, providing little or no short term scaleability. But for multicast the cost does not increase as the number of receivers goes up--it is O(1). The sender simply transmits a single data stream, and the network replicates it as needed to ensure that it reaches all interested receivers. It costs effectively the same amount to send a multicast data stream to a single recipient as it does to send it to a billion recipients.

Any Source Multicast

The first big step in Internet multicast was Steve Deering's 1989 Ph.D. thesis, "Scalable Multicast Routing Protocol." This laid out the basic ideas, and described, in abstract terms, the mechanisms needed to distribute IP multicast on the Internet. Deering and others put the theory of IP multicast into practice to launch the MBone, an experimental network-within-the-networks for Internet multicast.

In the MBone's "Any Source Multicasting" (ASM) model, multicast traffic is divided into "groups" which are numbered like IP addresses. When an application wants to "join" group G, it notifies its host operating system, which in turn communicates this request to routers on the local network with Internet Group Management Protocol (IGMP). These routers, in turn, talk to other routers on the Internet and set up distribution trees for the desired group. Once a distribution tree is established, the application's host will receive every packet on the entire Internet sent to group G.

Sending data to a multicast group is much simpler. In the spirit of the "connectionless" model of the Internet, no advance warning needs to be given before transmiting multicast data; a host simply puts the data on the local wire and routers are responsible for delivering it to anyone who has expressed an interest in it.

One of the problems the ASM faces is hooking senders up with receivers. Getting multicast data packets, which can appear anywhere on the Internet at any time, delivered to everyone who is interested in them is a complex problem. The solution currently in use involves what are called Rendezvous Points (RPs). RPs are spread throughout the Internet and each one is responsible for knowing all the active senders to all the groups on the entire Internet. This allows other routers to forget about all the multicast traffic they are not actively distributing. Unfortunately, requiring a single machine to keep state information for every single multicast transmitter on the Internet is just the sort of thing that fails to scale, and becomes a bottleneck on the Internet.

Another problem with ASM occurs when two different applications--or two different sets of users with the same application--happen to pick the same group address G. The result is what we would call, in electrical terms, "cross talk." However, allocating unused addresses is a manageable problem since one of the first multicast applications created was Session Directory Service (SDR). SDR carries announcements for multimedia multicast sessions throughout the MBone. But SDR does nothing to prevent someone from maliciously flooding a multicast group with noise that "drowns out" the legitimate users. If anything, SDR simply helps a black-hat hacker identify her denial-of-service target.

Source Specific Multicast

A solution to the problems of ASM is a system called Source Specific Multicast (SSM). With SSM, receivers can specify which senders they want to receive data from. The combination of a sender source address and a group "(S,G)" is called a "channel." With SSM, anyone trying to flood a channel with noise, has also to spoof a source IP address so she looks like the legitimate sender. However, all multicast routing protocols in use today require routers to check the source address of a packet before forwarding it. This check, fortunately, effectively removes the possibilities for multicast address spoofing.

The biggest advantage of SSM is that it doesnt use Rendezvous Points at all. In fact a router doing SSM only needs to know about the channels currently flowing through it. And in most cases this will be roughly proportional to the number and speed of the interfaces it has to manage. For this reason, SSM is generally considered "cheap" in terms of hardware and software resources.

Isp Support Of Multicast

Both ASM and SSM, unfortunately, incur support costs for ISPs. In Brad's experience working at a large backbone Internet Service Provider, he found that multicast issues typically require about twice as much work to debug versus traditional unicast issues. Much of the increased support cost is due to the fact that support staff, so far, are generally less familiar with multicast. But beyond this issue, there are a plethora of "stupid network tricks" which can disrupt multicast, yet cause no problems at all for unicast. Todays jungle of IPSEC tunnels, VPNs, NAT, and ever smarter link-layer switching hardware makes the potential for problems and the difficulty of debugging them greater than ever. But as multicast is becoming ever more popular at the LAN and enterprise level, more and more engineers are learning how to configure and debug IP multicast, and to avoid network situations that create problems for multicast.

The Future

With multicast, anyone with even a moderate Internet connection has enough bandwidth to run an Internet TV station. RFC3170 lists and categorizes several multicast applications, but with such a dramatic change in the cost of information delivery, it's probably the applications we have not even thought of yet that will have the most impact.

While multicast lowers the cost of distributing information dramatically, there are still some unsolved problems. Most people do not yet have access to IP multicast because their ISPs do not yet support it. In Part Two, we will disuss some basics of multicast routing and look at the Automatic Multicast Tunneling draft, which is a workaround for end users to get direct access to Internet multicast, using existing Intel infrastructure.

In part Three, we will discuss "reliable multicast" this is a wide open research area that has a lot of potential, and IETF and IRTF working groups. Reliability is a problem addressed in other context that looks easy until you look closely.


Intel CEO Craig R. Barrett gave a keynote address this February that discusses convergence of telecommunications, media, and Intel-platform servers. You can read it in full at:

Multicasting protocols are modularized in the sense Sean Maloney talks about in his talk at:

Specifically, multicast traffic is a way for heterogenous Intel-architecture devices to connect, while coping with--and channeling--the continued explosive growth of Internet traffic.

Intel routers support a variety of multicast protocols already. Our later installments discuss many of these protocols in greater detail. In the meanwhile, a good glossary of multicast-related acronyms can be found at:

These articles put a particular emphasis on the multimedia applications of multicasting. But the LANDesk software, developed by Intel, is a nice example of the use of these technologies for coordinated software distribution: