Multicast Gets a Makeover

New protocol makes multicasting easier

As bandwidth-hogging multimedia traffic swarms over the Internet, multicast — a technology designed to reduce Internet congestion — is undergoing a transformation that will make Webcasting far more efficient.

A new traffic management protocol, called Source-Specific Multicast, solves a key network configuration problem that has limited the adoption of traditional Internet Protocol multicast technology. While upgrading networks to support Source-Specific Multicast will take time, experts say, so many companies are behind the effort that its likely to take off and become a standard part of next-generation Internet networks.

Today, virtually all data sent across the Internet, such as e-mail messages and Web pages, is in unicast format, which means a separate stream of data is sent from the server to each receiver. For live Webcasts or for massive software updates, however, the unicast method of sending data is highly inefficient, because servers must track each separate data transfer, and routers in the network have to carry multiple copies of the same packets of data.

Multicast avoids this problem by sending out only a single stream of data at the source, then splitting it into multiple streams at each router until it reaches all the recipients. The resulting data stream looks like a tree: The root of the tree is the source computer sending the data, and the data streams fan out into branches at each router. This saves bandwidth — and thus, it also saves money.

"Many companies are finding it hard to deliver content by traditional means, because the delivery cost exceeds the value of the content," says Michael Luby, chief technology officer at Digital Fountain, a San Francisco start-up developing content distribution technology that works with multicast. "Multicast offers the promise of allowing delivery costs to scale according to the amount of content and not the size of the audience."

A downside of multicast — and something that has held up its adoption — is that every router in the network must support multicast in order to get the bandwidth-saving benefits. For now, multicast is most often used within corporate intranets. Some content delivery networks have multicast capabilities, so their distributed servers can broadcast content to one another, and a few Internet service providers, including Sprint and UUnet, offer multicast services to customers over their own backbone networks.

But across the public Internet, multicast is limited because most routers are not multicast-enabled. This is partly because ISPs havent upgraded legacy equipment, but the main issue, experts say, is that multicast has gotten a reputation for being nearly impossible to configure properly and debug.

"One of the main criticisms of multicast is that its so complicated — its hard to understand," says Kevin Almeroth, a professor of computer science at the University of California at Santa Barbara. "It forces you to think backwards."

The Rendezvous Point Problem

When a user requests access to a traditional multicast session, identified by a group number that has been broadcast to the entire network, the request is automatically relayed to a specially configured router, known as a rendezvous point. The source of the multicast is streaming data to the rendezvous point, and the end user is automatically connected to the appropriate tree for that multicast, with the rendezvous point at the root.

Once the user is linked to the right multicast tree, the rendezvous point is no longer necessary. The tree then reconfigures itself so that the multicast source becomes the root.

"The bad reputation [of multicast] is from when you have to configure these RPs [rendezvous points]," Almeroth says. "Any time you have to put functionality inside the network, its hard to do."

Source-Specific Multicast directly addresses this problem. With this protocol, both the group number for the multicast and the address of the source multicast server are broadcast to the network. This eliminates the need for the rendezvous point and makes the network far easier to configure.

The new Source-Specific Multicast protocol works just as well for Webcasts and the other multimedia applications that are driving the adoption of multicast, Almeroth says. Its only limitation is that it cant handle certain applications where the sources of data might be unknown, such as a videoconferencing application, where its not known in advance who will attend the session.

Source-Specific Multicast is expected to become an official, ratified specification of the Internet Engineering Task Force by midsummer. Meanwhile, most router vendors, including Cisco Systems and Juniper Networks, already support Source-Specific Multicast in their routers.

Some multimedia player software, such as that of RealNetworks and Microsoft, also support Source-Specific Multicast. The only piece missing is in the operating systems of personal computers, but thats changing, too. Various versions of Linux and Unix already support Source-Specific Multicast, and Microsoft, the worlds main provider of operating systems for personal computers, will support the new standard in Windows XP, due later this year.

Digital Fountains Luby says broad penetration of Source-Specific Multicast will probably take another two years. "Now we just have to wait for everyone to upgrade," he says.